Leetcode #46: Permutations

Leetcode_0046.cpp

class Solution {
    vector<vector<int>> answer;
    vector<int> numbers;
    
    void recurse(vector<int> path) {
        if (path.size() == numbers.size()) { // all numbers are added.
            answer.push_back(path); // add current permutation into answer.
        }
        
        for (int i=0; i<numbers.size(); i++) { // for each possibility.
            if (find(path.begin(), path.end(), numbers[i]) != path.end()) {
                // Adding same number twice would make the permutation 
                // invalid, so skip this number from consideration.
                continue;
            }
            path.push_back(numbers[i]); // consider this possibility.
            recurse(path); // recursively solve.
            path.pop_back(); // undo the considered possibility.
        }
    }
    
public:
    vector<vector<int>> permute(vector<int>& nums) {
        numbers = nums;
        vector<int> path; // to keep track of considered numbers 
                          // and their order. Initially empty.
        recurse(path);
        return answer;
    }
};