LeetCode

Permutations

2013.12.15 05:08

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

Solution:

　　The STL library <algorithm> provides a function next_permutation(), which generates the next greater permutation for a sequence. If no greater permutation exists, the smallest permutation will be returned as the result.

　　I just called next_permutation() for n! times, thus the time complexity is O(n * n!). next_permutation() requires O(n) time on average. Space complexity is O(n).

Accepted code:

// 1AC, next_permutation is handy~
#include <algorithm>
using namespace std;

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        for(int i = 0; i < result.size(); ++i){
            result[i].clear();
        }
        result.clear();

        int i;
        int n = num.size();
        int nn = 1;
        
        for(i = 1; i <= n; ++i){
            nn *= i;
        }
        
        for(i = 0; i < nn; ++i){
            result.push_back(vector<int>(num));
            next_permutation(num.begin(), num.end());
        }
        
        return result;
    }
private:
    vector<vector<int>> result;
};