LeetCode230: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?

求二叉搜索树的第k个最小的节点的值。

二叉搜索树的中序遍历结果就是它的从小到大的排列，所以可以参考中序遍历的思路。

使用非递归的解法可以查看中序遍历每次遍历的元素是那个，可以根据这一点来求解，对二叉搜索树进行中序遍历，每遍历到一个节点计数器加1，当它的值和k相等时，返回这个节点的值。

runtime:24ms

/**
 * 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 *> s;
        TreeNode * node=root;
        int pos=0;
        while(!s.empty()||node)
        {
            while(node)
            {
                s.push(node);
                node=node->left;
            }
            node=s.top();
            pos++;
            if(pos==k)
                return node->val;
            s.pop();
            node=node->right;
        }
    }
};