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.

Example 1:

Input: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
Output: 3

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?

分析

题目的意思是：找出二叉树中的第k小的值。
-解法当然是递归，中序遍历是左中右，刚好按照中序遍历，count到第k个就行了。

代码

/**
 * 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) {
        int ans=0;
        int count=0;
        solve(root,count,ans,k);
        return ans;
    }
    void solve(TreeNode* root,int& count,int& ans,int k){
        if(!root){
            return ;
        }
        solve(root->left,count,ans,k);
        count++;
        if(count==k){
            ans=root->val;
        }else if(count>=k){
            return ;
        }
        solve(root->right,count,ans,k);
    }
};

参考文献

​​230. Kth Smallest Element in a BST​​