[LeetCode] Kth Smallest Element in a BST

Kth Smallest Element in a BST

Given a binary search tree, write a functionkthSmallestto find thekth 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);}};

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