2023.10.21

        本题需要根据前序遍历序列和中序遍历序列来构造出一颗二叉树。类似于从中序与后序遍历序列构造二叉树 。使用递归， java代码如下：

/*** Definition for a binary tree node.* public 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;*     }* }*/
class Solution {public TreeNode buildTree(int[] preorder, int[] inorder) {if(inorder.length == 0) return null;int rootValue = preorder[0];TreeNode root = new TreeNode(rootValue);if(inorder.length == 1) return root;//找到中序遍历的切割点int seg = 0;for(int i=0; i<inorder.length; i++){if(inorder[i] == rootValue){seg = i;break;}}int[] in_left = Arrays.copyOfRange(inorder,0,seg);int[] in_right = Arrays.copyOfRange(inorder,seg+1,preorder.length);preorder = Arrays.copyOfRange(preorder,1,preorder.length);//去掉第一个元素，也就是根节点的值int[] pre_left = Arrays.copyOfRange(preorder,0,in_left.length);int[] pre_right = Arrays.copyOfRange(preorder,in_left.length,preorder.length);//递归处理左右子树root.left = buildTree(pre_left,in_left);root.right = buildTree(pre_right,in_right);return root;}
}