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:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        Stack<TreeNode> stack = new Stack<TreeNode>() ;
        TreeNode cur = root ;
        while(cur != null || !stack.isEmpty()){
            while(cur != null){
                stack.push(cur) ;
                cur =  cur.left ;
            }
            TreeNode node = stack.pop() ;
            if (--k == 0){
                return node.val ;
            }
            cur = node.right ;
        }
        return 0 ;
    }
}