Leetcode.230-Kth-Smallest-Element-In-A-Bst

二叉搜索树中第K小的元素

   使用中序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int i = 0;
    public int kthSmallest(TreeNode root, int k) {
        if(null == root){
            return 0;
        }
        
        int val = kthSmallest(root.left, k);
        if(i == k){
            return val;
        }
        i++;
        if(i == k){
            return root.val;
        }
        return kthSmallest( root.right,  k);
    }
}