Leetcode 145 Solution

This article provides solution to leetcode question 145 (binary-tree-postorder-traversal)
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https://leetcode.com/problems/binary-tree-postorder-traversal

Solution

/**
 * 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) {
        stack<std::pair<TreeNode*, int>> s;
        vector<int> res;

        if (root == NULL)
            return res;

        s.push(make_pair(root, 0));

        while (s.empty() == false)
        {
            auto pair = s.top();
            s.pop();

            auto node = pair.first;
            int stage = pair.second;

            if (node == NULL)
                continue;

            if (stage == 0)
            {
                s.push(make_pair(node, 1));
                s.push(make_pair(node->left, 0));
            }
            else if (stage == 1)
            {
                s.push(make_pair(node, 2));
                s.push(make_pair(node->right, 0));
            }
            else if (stage == 2)
            {
                res.push_back(node->val);
            }
        }

        return res;
    }
};