Kth Smallest Element in a BST

解题思路：按树的中序遍历的方式，利用栈实现，第k个出栈的节点即是所求的。

Java
代码实现：

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int kthSmallest(TreeNode root, int k) {
TreeNode p=new TreeNode(0);
p=root;
Stack sk=new Stack();
sk.push(p);
while(p.left!=null){
sk.push(p.left);
p=p.left;
}
int i=0;
while(!sk.isEmpty()){
TreeNode q=new TreeNode(0);
q=(TreeNode)sk.pop();
i++;
if(i==k) return q.val;
if(q.right!=null) {
p=q.right;
sk.push(p);
while(p.left!=null){
sk.push(p.left);
p=p.left;
}
}
}
return 0;
}
}

原题题目：https://leetcode.com/problems/kth-smallest-element-in-a-bst/