关于《剑指offer》某算法题的疑惑？

class Solution {
public:
    
    vector<vector<int> > res;//存放满足条件的所有路径
    vector<int> subRes; //存放某一条合格的路径
    
    vector<vector<int> > FindPath(TreeNode* root,int expectNumber) {        

      //vector<vector<int> > res;
       //vector<int> subRes; 

        if(root == NULL)
            return res;
        
        bool isLeaf = (root->left == NULL) && (root->right == NULL);//判断结点是否为叶子结点
        
        subRes.push_back(root->val);
        if((expectNumber - root->val) == 0 && isLeaf){
            res.push_back(subRes);
        }
        if(root->left)
            FindPath(root->left, expectNumber - root->val);
        if(root->right)
            FindPath(root->right, expectNumber - root->val);
        
        if(subRes.size())
            subRes.pop_back();
        
        return res;

    }
    
};