二叉树路径搜索---DFS 路径和

 vector<vector<int>>  pathSum(TreeNode* root,int sum){//DFS遍历获取适合路径，当递归到叶子结点且sum为0，表示该路径合适
 vector<vector<int>> ans;
 vector<int> path;

 helper(root,ans,path,sum);
 return ans;
 }
 void  helper(TreeNode* root,vector<vector<int>>& ans,vector<int>& path,int sun){
 if(root == NULL)   return;
 sum -= root->val;
 path.push_back(root->val);
 if(!root->right && !root->left && !sum)  ans.push_back(path);
 helper(root->left,ans,path,sum);
 helper(root->right,ans,path,sum);
 path.pop_back();//当该路径不合适，将末尾节点弹出，回溯到上一个节点有节点
 }