145-binary-tree-postorder-traversal.md

145. Binary Tree Postorder Traversal - 二叉树的后序遍历

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？

---

题目标签：Stack / Tree

题目链接：LeetCode / LeetCode中国

题解

Language	Runtime	Memory
cpp	8 ms	9.2 MB

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<pair<bool, TreeNode*>> stk;
        stk.push({false, root});
        while (!stk.empty()) {
            auto p = stk.top();
            stk.pop();
            bool flag = p.first;
            TreeNode* node = p.second;
            if (!node) continue;
            if (flag) {
                res.push_back(node->val);
            } else {
                stk.push({true, node});
                stk.push({false, node->right});
                stk.push({false, node->left});
            }
        }
        return res;
    }
};