bst_postorder_traversal.cpp

/* 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) {
      vector<int> preOrderT;
        stack<TreeNode*> st;

        if(!root)
            return preOrderT;

        st.push(root);

        while(!st.empty())
        {
            TreeNode* temp = st.top();
            st.pop();

            preOrderT.push_back(temp->val);

            if(temp->left)
                st.push(temp->left);

            if(temp->right)
                st.push(temp->right);
        }
        reverse(preOrderT.begin(), preOrderT.end());
        return preOrderT;
    }
};