145.binary-tree-postorder-traversal.cpp

/*
 * @lc app=leetcode id=145 lang=cpp
 *
 * [145] Binary Tree Postorder Traversal
 *
 * https://leetcode.com/problems/binary-tree-postorder-traversal/description/
 *
 * algorithms
 * Hard (49.48%)
 * Likes:    993
 * Dislikes: 50
 * Total Accepted:    276.1K
 * Total Submissions: 557.9K
 * Testcase Example:  '[1,null,2,3]'
 *
 * Given a binary tree, return the postorder traversal of its nodes' values.
 * 
 * Example:
 * 
 * 
 * Input: [1,null,2,3]
 * ⁠  1
 * ⁠   \
 * ⁠    2
 * ⁠   /
 * ⁠  3
 * 
 * Output: [3,2,1]
 * 
 * 
 * Follow up: Recursive solution is trivial, could you do it iteratively?
 * 
 */
/**
 * 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> ret;
        TreeNode* p = root;
        stack<TreeNode*> s;
        s.push(p);
        while (!s.empty()) {
            p = s.top();
            s.pop();
            if (p) {
                ret.push_back(p -> val);
                s.push(p -> left);
                s.push(p -> right);
            }
        }
        reverse(ret.begin(), ret.end());
        return ret;
    }
};