Kth Smallest Element in a BST Solutions in C++

// Source : https://leetcode.com/problems/kth-smallest-element-in-a-bst/
// Author : Hao Chen
// Date   : 2015-07-03




/**
 * 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:
    // in-order travel - recursive way
    int kthSmallestHelper_recursive(TreeNode* root, int& k) {
        if (root==NULL) return 0; //this behavior is undefined!
        
        //in-order travel
        int result = kthSmallestHelper_recursive(root->left, k);
        if (k==0) return result;
        
        k--;
        if (k==0) return root->val;
        
        
        return kthSmallestHelper_recursive(root->right, k);
    }
    // in-order travel - non-recursive way
    int kthSmallestHelper_nonRecursive(TreeNode* root, int k){
        stack<TreeNode*> s;
        
        while(!s.empty() || root){
            
            while (root) { 
                s.push(root);
                root = root->left;
            }
            
            k--;
            root = s.top()->right;

            if (k==0) return s.top()->val;
            
            s.pop();
        }
        return -1;
    }
    
    int kthSmallest(TreeNode* root, int k) {
        //return kthSmallestHelper_nonRecursive(root, k);
        return kthSmallestHelper_recursive(root, k);
    }
};

/// Source : https://leetcode.com/problems/kth-smallest-element-in-a-bst/description/
/// Author : liuyubobobo
/// Time   : 2018-07-29
#include <iostream>

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) {}
};

/// Inorder Traversal
/// Time Complexity: O(n)
/// Space Complexity: O(h) where h is the height of the BST
class Solution {

private:
    int index;

public:
    int kthSmallest(TreeNode* root, int k) {
        index = 0;
        return kthSmallestNode(root, k)->val;
    }

private:
    TreeNode* kthSmallestNode(TreeNode* node, int k){

        if(node == NULL)
            return NULL;

        TreeNode* res = kthSmallestNode(node->left, k);
        if(res) return res;

        index ++;
        if(index == k)
            return node;

        return kthSmallestNode(node->right, k);
    }
};


int main() {

    return 0;
}