剑指 Offer 54. 二叉搜索树的第k大节点

/**
 * 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 ans;

    int kthLargest(TreeNode* root, int k) {
        dfs(root, k);
        return ans;
    }

    void dfs(TreeNode* root, int& k) {
        if (!root || !k) return;
        
        dfs(root->right, k);
        
        k--;
        if (k == 0) ans = root->val;
        
        dfs(root->left, k);
    }
};