113. Path Sum II.cpp

//Runtime: 8 ms, faster than 96.37% of C++ online submissions for Path Sum II.
//Memory Usage: 20.5 MB, less than 10.12% of C++ online submissions for 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>> ans;
    int sum;
    
    void backtrack(TreeNode* cur, vector<int>& path){
        //cur must not be nullptr
        //when entering this function, cur is added in "path"
        if(!cur->left && !cur->right){
            // for(const int& e : path) cout << e << " ";
            // cout << endl;
            if(accumulate(path.begin(), path.end(), 0) == sum){
                ans.push_back(path);
            }
        }else{
            if(cur->left){
                path.push_back(cur->left->val);
                backtrack(cur->left, path);
                path.pop_back();
            }
            
            if(cur->right){
                path.push_back(cur->right->val);
                backtrack(cur->right, path);
                path.pop_back();
            }
        }
    }
    
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        if(!root) return vector<vector<int>>();
        
        this->sum = sum;
        
        vector<int> path = {root->val};
        backtrack(root, path);
        
        return ans;
    }
};