leetcode 230 Kth Smallest Element in a BST

230. Kth Smallest Element in a BST

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?

解题思路：

中序遍历即可

/** * 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 dp(TreeNode* root, int& k){        if(root==NULL)return 0;        if(root->left!=NULL){                  //左孩子不为空            int n=dp(root->left,k);            if(k<1)return n;                    }        if(k--==1)return root->val;            else return dp(root->right,k);    }    int kthSmallest(TreeNode* root, int k) {       return dp(root,k);    }};