路径总和 II

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    vector<vector<int>> res;
private:
    void pathSumCore(TreeNode* root,int sum,vector<int>& tmp)//加&，节省内存，递归结束要回溯
    {
        if(root==nullptr)
        {
            return ;
        }
        else if(root->left==nullptr&&root->right==nullptr)
        {
            if(root->val-sum==0)
            {
                tmp.push_back(root->val);
                res.push_back(tmp);
                tmp.pop_back();//回溯
            }
            return ;
        }

        sum-=root->val;
        tmp.push_back(root->val);
        pathSumCore(root->left,sum,tmp);
        pathSumCore(root->right,sum,tmp);
        tmp.pop_back();//回溯
    }
public:
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        if(root==nullptr)
            return {};
        
        vector<int> tmp;
        pathSumCore(root,sum,tmp);
        return res;
    }
};