LeetCode "Find Leaves of Binary Tree"

/**
 * 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 {
    vector<vector<int>> ret;
    int go(TreeNode *p) // return max height
    {
        if(!p) return 0;
        
        int hl = go(p->left);
        int hr = go(p->right);
        
        int h = max(hl, hr) + 1;
        if(h > ret.size()) ret.push_back({});
        ret[h - 1].push_back(p->val);
        
        return h;
    }
public:
    vector<vector<int>> findLeaves(TreeNode* root) {
        go(root);
        return ret;
    }
};