LeetCode Kth Smallest Element In A BST

Description

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note:
You may assume k is always valid, 1 ≤ k ≤ BST’s total elements.

The original problem is here.

The original code is here.

My Solution

I solve this problem in C++, as below:

/*
*Kth Smallest Element in a BST 
*Author: shuaijiang
*Email: zhaoshuaijiang8@gmail.com
*/
#include<iostream>
#include<stack>
#include<stdlib.h>
using namespace std;

/**
 * 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:
    int kthSmallest(TreeNode* root, int k) {
        stack<TreeNode*> myStack;
        vector<int> result;
        if(root==NULL)
            return 0;
        myStack.push(root);
        
        while(!myStack.empty()){
            TreeNode * node = myStack.top();
            myStack.pop();
            if(node->left == NULL && node->right == NULL)
                result.push_back(node->val);
            else{
                if(node->right != NULL)
                    myStack.push(node->right);
                TreeNode *left = node->left;
                node->left = NULL;
                node->right = NULL;
                myStack.push(node);
                if(left != NULL)
                    myStack.push(left);                               
            }
            if(result.size()==k)
                return result[k-1];
        }
    }
};

Note

To solve the problem, I traversal the BST use a stack, and pop the stack from samll element to large. Finally, return the kth smallest element.