[LeetCode] Kth Smallest Element in a BST

Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallestto 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);
    }
};