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• 原题连接：101. 对称二叉树

1- 思路

借助队列

• 1- 创建队列：Queue<TreeNode> queue，初始化加入 root.left 和 root.right
• 2- 判断逻辑：while(!queue.isEmpty())
  • 2-1 弹出两个结点： 分别为 l 和 r
    • ① 情况1：l == null 且 r ==null 此时，直接 continue
    • ② 情况2：l == null 且 r !=null 此时，直接 返回 false
    • ③ 情况3：l != null 且 r ==null 此时，直接 返回 false
    • ④ 情况4：l.val != r.val 直接返回 false
  • 2-2 入栈顺序
    • 先入栈 l.left ，再入栈 r.right
    • 先入栈 l.right，再入栈 r.left

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2- 实现

⭐101. 对称二叉树——题解思路

class Solution {public boolean isSymmetric(TreeNode root) {Queue<TreeNode> queue = new LinkedList<>();queue.offer(root.left);queue.offer(root.right);while(!queue.isEmpty()){TreeNode l = queue.poll();TreeNode r = queue.poll();// 四种if( l == null && r==null){continue;}if( l== null && r!=null ){return false;}if(l!=null && r == null){return false;}if(l.val!=r.val){return false;}queue.offer(l.left);queue.offer(r.right);queue.offer(l.right);queue.offer(r.left);}return true;}
}

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3- ACM 实现

public class isSymmetric {public static class TreeNode {int val;TreeNode left;TreeNode right;TreeNode() {}TreeNode(int val) { this.val = val; }TreeNode(int val, TreeNode left, TreeNode right) {this.val = val;this.left = left;this.right = right;}}public static TreeNode build(String str){if(str == null || str.length()==0){return null;}String input = str.replace("[","");input = input.replace("]","");String[] parts = input.split(",");Integer[] nums = new Integer[parts.length];for(int i = 0 ; i < parts.length;i++){if(!parts[i].equals("null")){nums[i] = Integer.parseInt(parts[i]);}else{nums[i] = null;}}Queue<TreeNode> queue = new LinkedList<>();TreeNode root = new TreeNode(nums[0]);queue.offer(root);int index = 1;while(!queue.isEmpty()&& index<parts.length){TreeNode node = queue.poll();if(index<nums.length && nums[index]!=null){node.left = new TreeNode(nums[index]);queue.offer(node.left);}index++;if(index<nums.length && nums[index]!=null){node.right = new TreeNode(nums[index]);queue.offer(node.right);}index++;}return root;}public static boolean isSymetric(TreeNode root){Queue<TreeNode> queue = new LinkedList<>();queue.offer(root.left);queue.offer(root.right);while(!queue.isEmpty()){TreeNode l = queue.poll();TreeNode r = queue.poll();if(l==null && r==null){continue;}if(l!=null && r==null){return false;}if(l==null && r!=null){return false;}if(l.val!=r.val){return false;}queue.offer(l.left);queue.offer(r.right);queue.offer(l.right);queue.offer(r.left);}return true;}public static void main(String[] args) {Scanner sc = new Scanner(System.in);String input = sc.nextLine();TreeNode root = build(input);System.out.println("结果是"+isSymetric(root));}}