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]
]

class Solution {
public:
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        vector<vector<int>> result;
        vector<int> a;
        dfs(root,sum,a,result);
        return result;
    }
    void dfs(TreeNode* root,int sum,vector<int> &a,vector<vector<int>> &result)
    {
        if(root == NULL)
          return;
        a.push_back(root->val);
        if(root->left == NULL&&root->right == NULL)
          if(sum-root->val == 0)
            result.push_back(a);
        
        dfs(root->left,sum-root->val,a,result);
        dfs(root->right,sum-root->val,a,result);
        a.pop_back();
    }
};