leetcode-230-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?

Hint:

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

给一个BST,找出第K小的元素。

先遍历一次，按顺序存下来，再返回特定坐标即可。

/**
 * 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) {
            stack<TreeNode*> st;
            vector<int> ans;
            while(!st.empty()||root)
                {
                    while(root)
                        {
                            st.push(root);
                            root=root->left;
                        }
            TreeNode* tem=st.top();
            ans.push_back(tem->val);
            st.pop();
            if(tem->right)
                root=tem->right;

                    }
            return ans[k-1];

    }
    };