二叉树中和为某一值的路径

bool cmp(const vector<int> &a, const vector<int> &b){
    return a.size() > b.size();
}
class Solution {
public:
    vector<vector<int> >res;
    void dfs(TreeNode*root, int expNum, vector<int>vct){
        if(root == NULL) return;
        if(root->val > expNum){
            return;
        }
        if(root->left == NULL && root->right == NULL && root->val == expNum){
            //找到一条路径
            vct.push_back(root->val);
            res.push_back(vct);
            vct.pop_back();
            return;
        }
        if(root->left != NULL){
            vct.push_back(root->val);
            dfs(root->left, expNum-root->val, vct);
            vct.pop_back();
        }
        if(root->right != NULL){
            vct.push_back(root->val);
            dfs(root->right, expNum-root->val, vct);
            vct.pop_back();
        }
    }

    vector<vector<int> > FindPath(TreeNode* root,int expectNumber) {
        vector<int> vct;
        dfs(root, expectNumber, vct);
        sort(res.begin(), res.end(), cmp);
        return res;
    }
};