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leetcode 145 Solution

发表于 更新于 分类于 150 1 分钟

代码解析

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package com.demo.s145;

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;

/**
 * 二叉树的后序遍历
 * 给你一棵二叉树的根节点 root ，返回其节点值的 后序遍历 。
 */
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;
    }
}
public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        //返回值
        List<Integer> res = new ArrayList<Integer>();
        if (root == null) {
            return res;
        }
        //新建栈
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
        TreeNode prev = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                //先左孩子入栈
                root = root.left;
            }
            root = stack.pop();
            if (root.right == null || root.right == prev) {
                res.add(root.val);
                prev = root;
                root = null;
            } else {
                stack.push(root);
                //右孩子入栈
                root = root.right;
            }
        }
        return res;
    }
}