[Leetcode] Path Sum II

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

Analysis:

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:

    void helper(TreeNode *root, int sum, vector<vector<int> >& ret, vector<int>& nums)
    {
        if (root == NULL)
            return;
        //have to be leaft
        else if (root->left == NULL && root->right == NULL && sum == root->val)
        {
            //leaf number was not pushed yet
            nums.push_back(root->val);
            ret.push_back(nums);
            nums.pop_back();
        }
        else
        {
            nums.push_back(root->val);
            helper(root->left, sum - root->val, ret, nums);
            helper(root->right, sum - root->val, ret, nums);
            nums.pop_back();
        }
    }

    vector<vector<int> > pathSum(TreeNode *root, int sum) {
        vector<vector<int> > ret;
        vector<int> nums;
        helper(root, sum, ret, nums);
        
        return ret;
    }
};