[LeetCode] Kth Smallest Element in a BST

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：

这道题求的是二分查找树种第k大的数。二分查找树有一个特点，每个节点均大于左孩子树的所有节点，小于有孩子树的所有节点。

因此可以利用中序遍历的方法，扫描二分查找树，当扫描到第k个数时，停止继续扫描。

/**
* 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 count = 0;
int result = 0;
inOrder(root, k, count, result);
return result;
}

void inOrder(TreeNode* root, int k, int& count, int& result){
if(root==NULL || count>=k){
return;
}
inOrder(root->left, k, count, result);
count++;
if(count==k){
result=root->val;
}
inOrder(root->right, k, count, result);
}
};