113.path-sum-ii 路径总和 II

#include <vector>
using std::vector;
class Solution {
  private:
    void get_sum(TreeNode *root, vector<int> path, int sum, vector<vector<int>> &res, int targetSum) {
        sum += root->val;//回溯被隐藏起来了
        path.push_back(root->val);//回溯被隐藏起来了，因为函数里的path改变了，没有改变函数外的path
        if (root->left == nullptr && root->right == nullptr) {
            if (sum == targetSum) {
                res.push_back(path);
                return;
            }
            else
                return;    
        }
        if (root->left != nullptr) 
            get_sum(root->left, path, sum, res, targetSum);
        if (root->right != nullptr)
            get_sum(root->right, path, sum, res, targetSum);
        return;
    }
  public:
    vector<vector<int>> pathSum(TreeNode *root, int targetSum) {
        vector<vector<int>> res;
        vector<int> path;
        if (root == nullptr)
            return res;
        get_sum(root, path, 0, res, targetSum);
        return res;
    }
};