https://leetcode.cn/problems/path-sum-ii/
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> ret;
    vector<int> path;
    int target = 0;
    int sum = 0;

    vector<vector<int>> pathSum(TreeNode* root, int targetSum) {
        target = targetSum;
        dfs(root);

        return ret;
    }

    void dfs(TreeNode* root)
    {
        if(root == nullptr)
            return;

        sum += root->val;
        path.push_back(root->val);
        
        if(root->left == nullptr && root->right == nullptr && sum == target)
        {
            ret.push_back(path);
        }

        dfs(root->left);
        path.pop_back();
        sum -= root->val;
        
        sum += root->val;
        path.push_back(root->val);
        dfs(root->right);
        path.pop_back();
        sum -= root->val;
    }
};