LeetCode - Permutations

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:

 1 // 1AC, next_permutation is handy~

 2 #include <algorithm>

 3 using namespace std;

 4 

 5 class Solution {

 6 public:

 7     vector<vector<int> > permute(vector<int> &num) {

 8         // IMPORTANT: Please reset any member data you declared, as

 9         // the same Solution instance will be reused for each test case.

10         for(int i = 0; i < result.size(); ++i){

11             result[i].clear();

12         }

13         result.clear();

14 

15         int i;

16         int n = num.size();

17         int nn = 1;

18         

19         for(i = 1; i <= n; ++i){

20             nn *= i;

21         }

22         

23         for(i = 0; i < nn; ++i){

24             result.push_back(vector<int>(num));

25             next_permutation(num.begin(), num.end());

26         }

27         

28         return result;

29     }

30 private:

31     vector<vector<int>> result;

32 };