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.

Example 1:

Input: root = [3,1,4,null,2], k = 1
   3
  / 
 1   4
  
   2
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / 
     3   6
    / 
   2   4
  /
 1
Output: 3

二叉搜索树的第k小节点

java:

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public int kthSmallest(TreeNode root, int k) {
12         Stack<TreeNode> stack = new Stack<TreeNode>() ;
13         TreeNode cur = root ;
14         while(cur != null || !stack.isEmpty()){
15             while(cur != null){
16                 stack.push(cur) ;
17                 cur =  cur.left ;
18             }
19             TreeNode node = stack.pop() ;
20             if (--k == 0){
21                 return node.val ;
22             }
23             cur = node.right ;
24         }
25         return 0 ;
26     }
27 }