描述：

給定一個二叉樹，找出所有路徑中各節點相加總和等於給定 目標值 的路徑。

一個有效的路徑，指的是從根節點到葉節點的路徑。

樣例：

給定一個二叉樹，和 目標值 = 5:

     1
    / \
   2   4
  / \
 2   3

返回：

[
  [1, 2, 2],
  [1, 4]
]

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root the root of binary tree
     * @param target an integer
     * @return all valid paths
     */
    vector<vector<int>> va;
    void pathsum(TreeNode *root,int sum,int goal,vector<int> v)
    {
        
        if(root->left==NULL && root->right==NULL)
        {
            if(sum==goal)
            va.push_back(v);
            return;
        }
        
        int s;
        
        if(root->left!=NULL)
        {
            vector<int> vm(v);
            s=sum+root->left->val;
            vm.push_back(root->left->val);
            pathsum(root->left,s,goal,vm);
        }
        if(root->right!=NULL)
        {
            vector<int> vm(v);
            s=sum+root->right->val;
            vm.push_back(root->right->val);
            pathsum(root->right,s,goal,vm);
        }
    }
    vector<vector<int>> binaryTreePathSum(TreeNode *root, int target) {
        // Write your code here
        if(root==NULL)
        return va;
        vector<int>v;
        v.push_back(root->val);
        pathsum(root,root->val,target,v);
        return va;
    }
};