二叉树

二叉搜索树的中序遍历是递增序列。

/**
 * 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 ret,k;
    void dfs(TreeNode root){
        if(root==null) return;
        dfs(root.left);
        if(k==0) return;
        if(--k==0) ret=root.val;
        dfs(root.right);
    }
    public int kthSmallest(TreeNode root, int k) {
         this.k=k;
         dfs(root);
         return ret;
    }
}