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;
    }
}