二叉搜索树的第k大问题之中序遍历的倒序

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int kthLargest(TreeNode* root, int k) {
        dfs(root,k);
        return num;
    }
    void dfs(TreeNode* node,int& k){
        if(node == nullptr) return;
        if(node->right) dfs(node->right,k);//先遍历右子节点
        if(--k == 0){
            num = node->val;//当前根节点；
            return;
        }
        if(node->left) dfs(node->left,k);//遍历左子节点
    }
private:
    int num;
};