145. 二叉树的后序遍历

Posted 2021-12-02 tu9oh0st

题目描述

给定一个二叉树，返回它的 后序 遍历。

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？

分析

后序遍历顺序是 left->right->root

贴出代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    // 迭代版
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        LinkedList<TreeNode> s = new LinkedList<>();
        TreeNode current = root, isVisited = null;
        while (current != null || !s.isEmpty()){
            while (current != null){
                s.push(current);
                current = current.left;
            }
            current = s.peek();
            if (current.right == null || current.right == isVisited){
                s.pop();
                res.add(current.val);
                isVisited = current;
                current = null;
            }else {
                current = current.right;
            }
        }
        return res;
    }
}