[LeetCode230] Kth Smallest Element in a BST

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:

分类：Tree Binary Search

代码：

第一种：需要额外空间，不太好

class Solution {
private:
    vector<int> res;
public:
    int kthSmallest(TreeNode* root, int k) {
        
        if(!root)
            return 0;
        inorder(root);
        return res[k - 1];
    }
    
    void inorder(TreeNode* root)
    {
        if(root)
        {
            inorder(root->left);
            res.push_back(root->val);
            inorder(root->right);
        }
    }
};

第二种：

class Solution {

public:
    int kthSmallest(TreeNode* root, int k) {
        if(!root)
            return 0;
        //非递归中序遍历
        stack<TreeNode*> stk;
        TreeNode* cur = root;
        while(cur || !stk.empty())
        {
            while(cur)
            {
                stk.push(cur);
                cur = cur->left;
            }
            cur = stk.top();
            if(--k == 0)
                return cur->val;
            stk.pop();
            cur = cur->right;
        }
        return  0;
    }
};