Kth Smallest Element in a BST

Total Accepted: 28176 Total Submissions: 82688 Difficulty: Medium

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.

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

 (M) Binary Tree Inorder Traversal

 

/**
 * 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:
    void inorder(TreeNode* root, int &k,int &val){
        if(!root) return;
        inorder(root->left,k,val);
        if(--k == 0){
            val = root->val;
            return;
        }
        inorder(root->right,k,val);
    }
    int kthSmallest(TreeNode* root, int k) {
        int res = 0;
        inorder(root,k,res);
        return res;
    }
};

Next challenges: (M) Convert Sorted Array to Binary Search Tree (M) Path Sum II (H) Dungeon Game