1. 题目

2. 思路

(1) 递归

• 后序遍历的最后一个结点是根结点，在中序遍历中定位根结点的位置，则中序遍历中根结点的左边均为左子树结点，右边均为右子树结点，递归构建左右子树即可。
• 定位根结点的位置可以利用HashMap。

3. 代码

import java.util.HashMap;
import java.util.Map;

public class Test {
    
    
    public static void main(String[] args) {
    
    
    }
}

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 {
    
    
    private int[] postorder;
    private Map<Integer, Integer> map;

    public TreeNode buildTree(int[] inorder, int[] postorder) {
    
    
        this.postorder = postorder;
        map = new HashMap<>();
        for (int i = 0; i < inorder.length; i++) {
    
    
            map.put(inorder[i], i);
        }
        return recur(0, inorder.length - 1, 0, postorder.length - 1);
    }

    private TreeNode recur(int inLeft, int inRight, int postLeft, int postRight) {
    
    
        if (inLeft > inRight) {
    
    
            return null;
        }
        TreeNode root = new TreeNode(postorder[postRight]);
        int mid = map.get(postorder[postRight]);
        int leftCount = mid - inLeft;
        root.left = recur(inLeft, inLeft + leftCount - 1, postLeft, postLeft + leftCount - 1);
        root.right = recur(mid + 1, inRight, postLeft + leftCount, postRight - 1);
        return root;
    }
}