dataType[] arrayRefVar;	//首选的方法// 或dataType arrayRefVar[];	//效果相同，但不是首选方法

  dataType[ ] arrayRefVar = new dataType[ arraySize];

public class ArrayList<E> extends AbstractList<E>implements List<E>, RandomAccess, Cloneable, java.io.Serializable {//可序列化版本号private static final long serialVersionUID = 8683452581122892189L;//默认的初始化数组大小 为10 .private static final int DEFAULT_CAPACITY = 10;//实例化一个空数组private static final Object[] EMPTY_ELEMENTDATA = {};//存放List元素的数组private transient Object[] elementData;//List中元素的数量，和存放List元素的数组长度可能相等，也可能不相等private int size;//构造方法，指定初始化的数组长度public ArrayList(int initialCapacity) {super();if (initialCapacity < 0)throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);this.elementData = new Object[initialCapacity];}//无参构造方法public ArrayList() {super();this.elementData = EMPTY_ELEMENTDATA;}//构造方法，参数为集合元素public ArrayList(Collection<? extends E> c) {//将集合转换成数组，并赋值给elementData数组elementData = c.toArray();size = elementData.length;//如果c.toArray返回的不是Object[]类型的数组，转换成Object[]类型if (elementData.getClass() != Object[].class)elementData = Arrays.copyOf(elementData, size, Object[].class);}//改变数组的长度，使长度和List的size相等。public void trimToSize() {modCount++;if (size < elementData.length) {elementData = Arrays.copyOf(elementData, size);}}//确定ArrayList的容量//判断当前elementData是否是EMPTY_ELEMENTDATA,若是设置长度为10public void ensureCapacity(int minCapacity) {int minExpand = (elementData != EMPTY_ELEMENTDATA)// any size if real element table? 0// larger than default for empty table. It's already supposed to be// at default size.: DEFAULT_CAPACITY;//是否需要扩容if (minCapacity > minExpand) {ensureExplicitCapacity(minCapacity);}}//当前位置和默认大小之间取最大值private void ensureCapacityInternal(int minCapacity) {if (elementData == EMPTY_ELEMENTDATA) {minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);}private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}//数组的最大容量private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;//扩容操作private void grow(int minCapacity) {// overflow-conscious codeint oldCapacity = elementData.length;//容量扩充1.5倍int newCapacity = oldCapacity + (oldCapacity >> 1);if (newCapacity - minCapacity < 0)newCapacity = minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);// minCapacity is usually close to size, so this is a win://生成一个长度为newCapacity数组，并将elementData数组中元素拷贝到新数组中，并将新数组的引用赋值给elementDataelementData = Arrays.copyOf(elementData, newCapacity);}private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}//返回数组中已经放入的元素个数，非数组长度public int size() {return size;}//List是否为空public boolean isEmpty() {return size == 0;}//判断是否包包含指定元素public boolean contains(Object o) {return indexOf(o) >= 0;}//查找指定的元素，存在返回下标，不存在放回 -1 public int indexOf(Object o) {if (o == null) {for (int i = 0; i < size; i++)if (elementData[i]==null)return i;} else {for (int i = 0; i < size; i++)if (o.equals(elementData[i]))return i;}return -1;}//倒序查找元素，存在放回下标，不存在返回-1 public int lastIndexOf(Object o) {if (o == null) {for (int i = size-1; i >= 0; i--)if (elementData[i]==null)return i;} else {for (int i = size-1; i >= 0; i--)if (o.equals(elementData[i]))return i;}return -1;}//因为实现了clone接口，所以需要重写clone()方法，实现对象的拷贝public Object clone() {try {@SuppressWarnings("unchecked")ArrayList<E> v = (ArrayList<E>) super.clone();v.elementData = Arrays.copyOf(elementData, size);v.modCount = 0;return v;} catch (CloneNotSupportedException e) {// this shouldn't happen, since we are Cloneablethrow new InternalError();}}//将集合转化为数组public Object[] toArray() {return Arrays.copyOf(elementData, size);}//转化为指定类型的数组元素，推荐使用此方法@SuppressWarnings("unchecked")public <T> T[] toArray(T[] a) {if (a.length < size)// Make a new array of a's runtime type, but my contents:return (T[]) Arrays.copyOf(elementData, size, a.getClass());System.arraycopy(elementData, 0, a, 0, size);if (a.length > size)a[size] = null;return a;}// Positional Access Operations//放回指定位置的数组元素@SuppressWarnings("unchecked")E elementData(int index) {return (E) elementData[index];}//返回列表中指定位置的元素public E get(int index) {rangeCheck(index);return elementData(index);}//设置指定位置的元素，并返回被替换的元素public E set(int index, E element) {rangeCheck(index);E oldValue = elementData(index);elementData[index] = element;return oldValue;}//添加元素public boolean add(E e) {ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}//将元素添加到指定位置上，从指定位置的元素开始所有元素向后移动，为新添加的元素提供位置public void add(int index, E element) {rangeCheckForAdd(index);ensureCapacityInternal(size + 1);  // Increments modCount!!System.arraycopy(elementData, index, elementData, index + 1,size - index);elementData[index] = element;size++;}//删除指定位置的元素，其他元素做相依的移动，并将最后一个元素置空，方便垃圾处理机制回收，防止内存泄露，并返回删除的元素值public E remove(int index) {rangeCheck(index);modCount++;E oldValue = elementData(index);int numMoved = size - index - 1;if (numMoved > 0)System.arraycopy(elementData, index+1, elementData, index,numMoved);elementData[--size] = null; // clear to let GC do its workreturn oldValue;}//删除元素方法public boolean remove(Object o) {if (o == null) {for (int index = 0; index < size; index++)if (elementData[index] == null) {fastRemove(index);return true;}} else {for (int index = 0; index < size; index++)if (o.equals(elementData[index])) {fastRemove(index);return true;}}return false;}//快速删除执行操作private void fastRemove(int index) {modCount++;int numMoved = size - index - 1;if (numMoved > 0)System.arraycopy(elementData, index+1, elementData, index,numMoved);elementData[--size] = null; // clear to let GC do its work}//清除列表public void clear() {modCount++;// clear to let GC do its workfor (int i = 0; i < size; i++)elementData[i] = null;size = 0;}//添加方法，添加的元素为集合public boolean addAll(Collection<? extends E> c) {Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew);  // Increments modCountSystem.arraycopy(a, 0, elementData, size, numNew);size += numNew;return numNew != 0;}//从指定位置开始添加集合元素public boolean addAll(int index, Collection<? extends E> c) {rangeCheckForAdd(index);Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew);  // Increments modCountint numMoved = size - index;if (numMoved > 0)System.arraycopy(elementData, index, elementData, index + numNew,numMoved);System.arraycopy(a, 0, elementData, index, numNew);size += numNew;return numNew != 0;}//范围删除方法protected void removeRange(int fromIndex, int toIndex) {modCount++;int numMoved = size - toIndex;System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);// clear to let GC do its workint newSize = size - (toIndex-fromIndex);for (int i = newSize; i < size; i++) {elementData[i] = null;}size = newSize;}//下标检测方法，如果不合法，抛出IndexOutOfBoundsException异常private void rangeCheck(int index) {if (index >= size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}/*** A version of rangeCheck used by add and addAll.*/private void rangeCheckForAdd(int index) {if (index > size || index < 0)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}//溢出信息private String outOfBoundsMsg(int index) {return "Index: "+index+", Size: "+size;}//删除所有元素public boolean removeAll(Collection<?> c) {return batchRemove(c, false);}public boolean retainAll(Collection<?> c) {return batchRemove(c, true);}private boolean batchRemove(Collection<?> c, boolean complement) {final Object[] elementData = this.elementData;int r = 0, w = 0;boolean modified = false;try {for (; r < size; r++)if (c.contains(elementData[r]) == complement)elementData[w++] = elementData[r];} finally {// Preserve behavioral compatibility with AbstractCollection,// even if c.contains() throws.if (r != size) {System.arraycopy(elementData, r,elementData, w,size - r);w += size - r;}if (w != size) {// clear to let GC do its workfor (int i = w; i < size; i++)elementData[i] = null;modCount += size - w;size = w;modified = true;}}return modified;}//流操作方法，将对象写入输出流中private void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException{// Write out element count, and any hidden stuffint expectedModCount = modCount;s.defaultWriteObject();// Write out size as capacity for behavioural compatibility with clone()s.writeInt(size);// Write out all elements in the proper order.for (int i=0; i<size; i++) {s.writeObject(elementData[i]);}if (modCount != expectedModCount) {throw new ConcurrentModificationException();}}//流操作，读方法，将对象从流中取出private void readObject(java.io.ObjectInputStream s)throws java.io.IOException, ClassNotFoundException {elementData = EMPTY_ELEMENTDATA;// Read in size, and any hidden stuffs.defaultReadObject();// Read in capacitys.readInt(); // ignoredif (size > 0) {// be like clone(), allocate array based upon size not capacityensureCapacityInternal(size);Object[] a = elementData;// Read in all elements in the proper order.for (int i=0; i<size; i++) {a[i] = s.readObject();}}}//迭代方法，返回内部类实例public ListIterator<E> listIterator(int index) {if (index < 0 || index > size)throw new IndexOutOfBoundsException("Index: "+index);return new ListItr(index);}//迭代方法，返回内部类实例public ListIterator<E> listIterator() {return new ListItr(0);}//迭代方法，返回内部类实例public Iterator<E> iterator() {return new Itr();}//内部类，实现Iterator接口private class Itr implements Iterator<E> {int cursor;       // index of next element to returnint lastRet = -1; // index of last element returned; -1 if no suchint expectedModCount = modCount;//是否还有下一个元素，返回true or false public boolean hasNext() {return cursor != size;}//返回元素@SuppressWarnings("unchecked")public E next() {checkForComodification();int i = cursor;if (i >= size)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;   //获取外部类的elementData数组if (i >= elementData.length)throw new ConcurrentModificationException();cursor = i + 1;return (E) elementData[lastRet = i];}//删除元素public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.remove(lastRet);  //调用外部类删除方法，删除指定位置的元素cursor = lastRet;lastRet = -1;expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}}//省略了ListItr、SubList两个内部类
}

ArrayList(int initialCapacity);
ArrayList(); 
ArrayList(Collection<? extends E> c);

public boolean add(E e) {// 关键 -> 添加之前，校验容量ensureCapacityInternal(size + 1); // 修改 size，并在数组末尾添加指定元素elementData[size++] = e;return true;
}// 关键 -> minCapacity = size+1，即表示执行完添加操作后，数组中的元素个数 
private void ensureCapacityInternal(int minCapacity) {// 判断内部数组是否为空if (elementData == EMPTY_ELEMENTDATA) {// 设置数组最小容量（>=10）minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);
}
//接着会判断添加操作会不会导致内部数组的容量饱和
private void ensureExplicitCapacity(int minCapacity) {modCount++;// 判断结果为 true，则表示接下来的添加操作会导致元素数量超出数组容量if (minCapacity - elementData.length > 0){// 真正的扩容操作grow(minCapacity);}
}//实现真正的扩容公式与数组的拷贝
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;private void grow(int minCapacity) {int oldCapacity = elementData.length;// 关键-> 容量扩充公式int newCapacity = oldCapacity + (oldCapacity >> 1);// 针对新容量的一系列判断if (newCapacity - minCapacity < 0){newCapacity = minCapacity;}if (newCapacity - MAX_ARRAY_SIZE > 0){newCapacity = hugeCapacity(minCapacity);}// 关键 -> 复制旧数组元素到新数组中去elementData = Arrays.copyOf(elementData, newCapacity);
}private static int hugeCapacity(int minCapacity) {if (minCapacity < 0){throw new OutOfMemoryError();}return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
}

3.3 链表

3.3.1 什么是链表

public static int binarySearch(Integer[] srcArray, int des) {//定义初始最小、最大索引int start = 0;int end = srcArray.length - 1;//确保不会出现重复查找，越界while (start <= end) {//计算出中间索引值int middle = (end + start)>>>1 ;//防止溢出if (des == srcArray[middle]) {return middle;//判断下限} else if (des < srcArray[middle]) {end = middle - 1;//判断上限} else {start = middle + 1;}}//若没有，则返回-1return -1;
}

5.2 选择排序

public int[] selectSort(int[] array) {if (array == null || array.length < 2) {return array;}for (int i = 0; i < array.length - 1; i++) {int minIndex = i;for (int j = i + 1; j < array.length; j++) {if (array[j] < array[minIndex]) {minIndex = j;}}if (minIndex > i) {array[i] = array[i] ^ array[minIndex];array[minIndex] = array[i] ^ array[minIndex];array[i] = array[i] ^ array[minIndex];}}return array;
}

public int[] insertSort(int[] array) {if (array == null || array.length < 2) {return array;}for (int i = 1; i < array.length; i++) {int temp = array[i];int j = i - 1;while (j >= 0 && array[j] > temp) {array[j + 1] = array[j];j--;}array[j + 1] = temp;}return array;
}

public int[] shellSort(int[] array) {if (array == null || array.length < 2) {return array;}int gap = array.length >>> 1;while (gap > 0) {for (int i = gap; i < array.length; i++) {int temp = array[i];int j = i - gap;while (j >= 0 && array[j] > temp) {array[j + gap] = array[j];j = j - gap;}array[j + gap] = temp;}gap >>>= 1;}return array;
}

public class MaxHeap {/*** 排序数组*/private int[] nodeArray;/*** 数组的真实大小*/private int size;private int parent(int index) {return (index - 1) >>> 1;}private int leftChild(int index) {return (index << 1) + 1;}private int rightChild(int index) {return (index << 1) + 2;}private void swap(int i, int j) {nodeArray[i] = nodeArray[i] ^ nodeArray[j];nodeArray[j] = nodeArray[i] ^ nodeArray[j];nodeArray[i] = nodeArray[i] ^ nodeArray[j];}private void siftUp(int index) {//如果index处节点的值大于其父节点的值，则交换两个节点值，同时将index指向其父节点，继续向上循环判断while (index > 0 && nodeArray[index] > nodeArray[parent(index)]) {swap(index, parent(index));index = parent(index);}}private void siftDown(int index) {//左孩子的索引比size小，意味着索引index处的节点有左孩子，证明此时index节点不是叶子节点while (leftChild(index) < size) {//maxIndex记录的是index节点左右孩子中最大值的索引int maxIndex = leftChild(index);//右孩子的索引小于size意味着index节点含有右孩子if (rightChild(index) < size && nodeArray[rightChild(index)] > nodeArray[maxIndex]) {maxIndex = rightChild(index);}//如果index节点值比左右孩子值都大，则终止循环if (nodeArray[index] >= nodeArray[maxIndex]) {break;}//否则进行交换，将index指向其交换的左孩子或右孩子，继续向下循环，直到叶子节点swap(index, maxIndex);index = maxIndex;}}private void add(int value) {nodeArray[size++] = value;//构建大顶堆siftUp(size - 1);}private void extractMax() {/*将堆顶元素和最后一个元素进行交换此时并没有删除元素，而只是将size-1，剩下的元素重新构建成大顶堆*/swap(0, --size);//重新构建大顶堆siftDown(0);}public int[] heapSort(int[] array) {if (array == null || array.length < 2) {return array;}nodeArray = new int[array.length];for (int value : array) {add(value);}for (int ignored : array) {extractMax();}return nodeArray;}
}

public int[] mergeSort(int[] array) {if (array == null || array.length < 2) {return array;}return mergeSort(array, 0, array.length - 1);
}private int[] mergeSort(int[] array, int left, int right) {if (left < right) {//这里没有选择“(left + right) / 2”的方式，是为了防止数据溢出int mid = left + ((right - left) >>> 1);// 拆分子数组mergeSort(array, left, mid);mergeSort(array, mid + 1, right);// 对子数组进行合并merge(array, left, mid, right);}return array;
}private void merge(int[] array, int left, int mid, int right) {int[] temp = new int[right - left + 1];// p1和p2为需要对比的两个数组的指针，k为存放temp数组的指针int p1 = left, p2 = mid + 1, k = 0;while (p1 <= mid && p2 <= right) {if (array[p1] <= array[p2]) {temp[k++] = array[p1++];} else {temp[k++] = array[p2++];}}// 把剩余的数组直接放到temp数组中while (p1 <= mid) {temp[k++] = array[p1++];}while (p2 <= right) {temp[k++] = array[p2++];}// 复制回原数组for (int i = 0; i < temp.length; i++) {array[i + left] = temp[i];}
}

5.7 快速排序

<table style="text-align: center;"><tr><td rowspan="2">排序算法</td><td colspan="3">时间复杂度</td><td rowspan="2">空间复杂度</td><td rowspan="2">稳定性</td></tr><tr><td>平均情况</td><td>最好情况</td><td>最坏情况</td></tr><tr><td>冒泡排序</td><td>O(n<sup>2</sup>)</td><td>O(n)</td><td>O(n<sup>2</sup>)</td><td>O(1)</td><td>稳定</td></tr><tr><td>选择排序</td><td>O(n<sup>2</sup>)</td><td>O(n<sup>2</sup>)</td><td>O(n<sup>2</sup>)</td><td>O(1)</td><td>不稳定</td></tr><tr><td>插入排序</td><td>O(n<sup>2</sup>)</td><td>O(n)</td><td>O(n<sup>2</sup>)</td><td>O(1)</td><td>稳定</td></tr><tr><td>希尔排序</td><td colspan="3">取决于增量的选择</td><td>O(1)</td><td>不稳定</td></tr><tr><td>堆排序</td><td>O(nlog<sub>2</sub>n)</td><td>O(nlog<sub>2</sub>n)</td><td>O(nlog<sub>2</sub>n)</td><td>O(1)</td><td>不稳定</td></tr><tr><td>归并排序</td><td>O(nlog<sub>2</sub>n)</td><td>O(nlog<sub>2</sub>n)</td><td>O(nlog<sub>2</sub>n)</td><td>O(n)</td><td>稳定</td></tr><tr><td>快速排序</td><td>O(nlog<sub>2</sub>n)</td><td>O(nlog<sub>2</sub>n)</td><td>O(n<sup>2</sup>)</td><td>O(nlog<sub>2</sub>n)</td><td>不稳定</td></tr><tr><td>计数排序</td><td>O(n+k)</td><td>O(n+k)</td><td>O(n+k)</td><td>O(k)</td><td>稳定</td></tr><tr><td>桶排序</td><td colspan="3">取决于桶散列的结果和内部排序算法的时间复杂度</td><td>O(n+l)</td><td>稳定</td></tr><tr><td>基数排序</td><td>O(d*(n+r))</td><td>O(d*(n+r))</td><td>O(d*(n+r))</td><td>O(n+r)</td><td>稳定</td></tr>
</table>

1. 和为 s 的两个数字

class Solution {public int[] twoSum(int[] nums, int target) {int i=0, j=nums.length-1;while(i<j){int s=nums[i]+nums[j];if(s<target) i++;else if(s>target) j--;else return new int[] {nums[i], nums[j]};}return new int[0];}
}

class Solution {public int[] exchange(int[] nums) {int left=0,right=nums.length-1;int tmp;while(left<right){if(nums[left]%2==1) left++;else if(nums[right]%2==0) right--;else{tmp=nums[left];nums[left]=nums[right];nums[right]=tmp;left++;right--;}}return nums;}
}

import java.util.Scanner;public class Queue {public static void main(String[] args){ArrayQueue arrayQueue=new ArrayQueue(5);//4.方便测试所以没有定义太大char key=' ';//5.键盘交互，接收用户输入Scanner scanner=new Scanner(System.in);boolean loop=true;while(loop){System.out.println("s(show):显示队列");System.out.println("e(exit):退出程序");System.out.println("a(add):向队列添加元素");System.out.println("g(get):向队列取出元素");System.out.println("h(head):查看队列的头数据");key=scanner.next().charAt(0);if (key<97) key+=32; //实现大小写都可判断switch (key){case 's' :arrayQueue.showQueue();break;case 'a':System.out.println("请输入一个数字");arrayQueue.addQueue(scanner.nextInt());break;case 'g' :try {System.out.println("您取到的数据为："+ arrayQueue.getQueue());} catch (Exception e){System.out.println(e.getMessage());}break;case 'h' :try {System.out.println("当前的头数据为："+ arrayQueue.headQueue());} catch (Exception e){System.out.println(e.getMessage());}break;case 'e' :scanner.close();//关闭输入loop=false;break;}}System.out.println("**********程序已退出*********");}
}class ArrayQueue{//1.创建一个实现队列的类private int MaxSize;    //队列的最大存储容量private int [] array;   //要存储队列的数组private int front;   //取出时移动的指针,指向队列前的一个位置;private int rear;    //存储时移动的指针,指向队列当前存储到的位置（最后一个数据）;//创建队列public ArrayQueue(int arrMaxSize){//2.设置构造器MaxSize=arrMaxSize;rear=-1;front=-1;array=new int[MaxSize];}//判断队列是否空public boolean isEmpty(){//3.一些必要的方法return rear==front;}//判断队列是否满public boolean isFull(){return rear==MaxSize-1;}//添加数据public void addQueue(int q){if(isFull()) System.out.println("队列已满，不可添加数据！");else {array[++rear]=q;//java可以这么写System.out.println("添加数据成功");}}//取出数据public int getQueue(){if (!isEmpty()){return array[++front];}throw new RuntimeException("队列为空");//没有合适的返回值，用异常}//显示当前队列所有数据public void showQueue(){if (isEmpty()) {System.out.println("队列为空，无可打印数据");return;}for (int i=0;i<array.length;i++){System.out.printf("%d ",array[i]);}System.out.println();}//返回当前队列的头数据public int headQueue(){if (isEmpty()) throw new NumberFormatException("队列为空，无头数据");return array[front+1];}
}