230. Kth Smallest Element in a BST

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.

DFS搜索，遍历的时候更新全局变量count。
采用中序搜索，当count等于k时，将全局变量ans设置为root.val。
搜索完毕返回ans。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int ans;
    int count;
    public int kthSmallest(TreeNode root, int k) {
        ans = -1;
        count = 0;
        dfs(root,k);
        return ans;
    }
    
    private void dfs(TreeNode root, int k){
        if(root == null){
            return;
        }
        dfs(root.left,k);
        count++;
        if (k == count){
            ans = root.val;
        }
        dfs(root.right,k);
    }
}