L13_AlltraversalsCpp

// LC link : https://leetcode.com/problems/binary-tree-postorder-traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {

public:
    vector<int> postorderTraversal(TreeNode* root) {
        stack<pair<TreeNode*,int>> st; 
        st.push({root, 1}); 
        vector<int> pre, in, post;
        if(root == NULL) return post;

        while(!st.empty()) {
            auto it = st.top(); 
            st.pop(); 

            // this is part of pre
            // increment 1 to 2 
            // push the left side of the tree
            if(it.second == 1) {
                pre.push_back(it.first->val); 
                it.second++; 
                st.push(it); 

                if(it.first->left != NULL) {
                    st.push({it.first->left, 1}); 
                }
            }

            // this is a part of in 
            // increment 2 to 3 
            // push right 
            else if(it.second == 2) {
                in.push_back(it.first->val); 
                it.second++; 
                st.push(it); 

                if(it.first->right != NULL) {
                    st.push({it.first->right, 1}); 
                }
            }
            // don't push it back again 
            else {
                post.push_back(it.first->val); 
            }
        } 

        return post; 
    }
};