LeetCode 145. 二叉树的后序遍历(Binary Tree Postorder Traversal)

145. 二叉树的后序遍历

145. Binary Tree Postorder Traversal

题目描述

给定一个二叉树，返回它的 后序 遍历。

LeetCode145. Binary Tree Postorder Traversal

示例:

输入: [1,null,2,3]

   1
    \
     2
    /
   3

输出: [3,2,1]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

Java 实现

Iterative Solution

import java.util.LinkedList;
import java.util.List;
import java.util.Stack;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new LinkedList<>();
        Stack<TreeNode> stack = new Stack<>();
        if (root == null) {
            return result;
        }
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            result.add(0, cur.val);
            if (cur.left != null) {
                stack.push(cur.left);
            }
            if (cur.right != null) {
                stack.push(cur.right);
            }
        }
        return result;
    }
}

Recursive Solution

import java.util.LinkedList;
import java.util.List;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

class Solution {
    List<Integer> result = new LinkedList<>();

    public List<Integer> postorderTraversal(TreeNode root) {
        if (root == null) {
            return result;
        }
        postorderTraversal(root.left);
        postorderTraversal(root.right);
        result.add(root.val);
        return result;
    }
}

相似题目

• 94. 二叉树的中序遍历
• 590. N叉树的后序遍历

参考资料

• https://leetcode-cn.com/problems/binary-tree-postorder-traversal/
• https://leetcode.com/problems/binary-tree-postorder-traversal/