54. 二叉搜索树的第k大节点

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int res,k;
    public int kthLargest(TreeNode root, int k) {
        //中序遍历逆序，若为求第k小时正序
        this.k = k;
        midOrder(root);
        return res;
    }
    
    private void midOrder(TreeNode root){
        if(root == null){
            return ;
        }        
        //右子树
        midOrder(root.right);
        k--;
        if(k == 0){
            res = root.val;
            return;
        }
        //左子树
        midOrder(root.left);
    }
}