LeetCode *** 230. 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:

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?

Hint:

Show Hint

Try to utilize the property of a BST.Show
More Hint
What if you could modify the BST node's structure?Show
More Hint
The optimal runtime complexity is O(height of BST).

分析：
我用的是最low的非递归中序遍历。

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:
int kthSmallest(TreeNode* root, int k) {

vector<int> res;
stack<TreeNode*> tree;
stack<bool> visited;

tree.push(root);
visited.push(false);

int count=0;

while(!tree.empty()){
TreeNode *tmp=tree.top();
bool isV=visited.top();
visited.pop();

if(tmp->left&&!isV){
tree.push(tmp->left);
isV=true;
visited.push(isV);
visited.push(false);
continue;
}

res.push_back(tmp->val);
tree.pop();

if(tmp->right){
tree.push(tmp->right);
visited.push(false);
}
}

return res[k-1];

}
};