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>> res;
    void dfs(TreeNode* root,int sum,vector<int> v){
        if(root==NULL) return;
        v.push_back(root->val);
        if(root->left==NULL&&root->right==NULL&&root->val==sum){ 
            res.push_back(v);
            return;
        }
        if(root->left){
            dfs(root->left,sum-root->val,v);
        }
        if(root->right){
            dfs(root->right,sum-root->val,v);
        }
    }
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        if(root==NULL) return res;
        dfs(root,sum,vector<int> (0));
        return res;
    }
};