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?

这道题思路比较明显，因为是二叉排序树，所以左子树的节点大小比根节点小，根节点的节点值大小又比右节点小，所以直接给出先序遍历，就是按照树中元素节点值得大小来排序的，代码见下程序。

/**
 * 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) {
        vector<int> res;
        LeftB(root,res);
        return res[k-1];
    }
    void LeftB(TreeNode* root,vector<int>& res){
        if(root->left)LeftB(root->left,res);
        res.push_back(root->val);
        if(root->right)LeftB(root->right,res);
    }
};