LeetCode - Path Sum II

解法一：recursion

class Solution {
public:
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        vector<vector<int>> res;
        vector<int> out;
        helper(root, sum, out, res);
        return res;
    }
    void helper(TreeNode* root, int sum, vector<int>& out, vector<vector<int>>& res){
        if(!root) return;
        out.push_back(root->val);
        if(!root->right && !root->left && sum==root->val) res.push_back(out);
        helper(root->left, sum-root->val, out, res);
        helper(root->right, sum-root->val, out, res);
        out.pop_back();
    }
};

解法二：non-recursion