[算法分析与设计] leetcode 每周一题: Kth Smallest Element in a BST

题目链接：https://leetcode.com/problems/kth-smallest-element-in-a-bst/description/

题目:

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?

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

思路：

维护一个动态数组，每次深搜树时，压入当前节点值，同时排序，如果数组大小大于K，则弹出最后一个，维持数组大小为K的状态，最后返回数组最后一个元素

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:
    vector<int> items;
    void findKthSmallest(TreeNode* root, int k) {
        if(root == nullptr) return ;
        items.push_back(root->val);
        sort(items.begin(),items.end());
        if(items.size() > k) {
            
            items.pop_back();
        }
        findKthSmallest(root->left,k);
        findKthSmallest(root->right,k);

    }

    int kthSmallest(TreeNode* root, int k) {
    
        findKthSmallest(root, k);
        int last = items.back();
        return last;
    }
};