[Leetcode] Permutations

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].

与Next Permutation一样，n个元素的排列共有n！个，所以只要执行n！次next_permutation就行。

class Solution {
public:
    bool nextPermutation(vector<int> &num) {
        if (num.size() < 2) {
            return false;
        }
        int i, k;
        bool flag = true;
        for (i = num.size() - 2; i >= 0; --i) {
            if (num[i] < num[i + 1]) {
                break;
            }
        }
        if (i < 0) {
            flag = false;
        }
        for (k = num.size() - 1; k > i; --k) {
            if (num[k] > num[i]) {
                break;
            }
        }
        swap(num[i], num[k]);
        reverse(num.begin() + i + 1, num.end());
        return flag;
    }
    
    long long getNum(int n) {
        long long res = 1;
        while (n > 0) {
            res *= n;
            n--;
        }
        return res;
    }
    
    vector<vector<int> > permute(vector<int> &num) {
        vector<vector<int> > res;
        res.push_back(num);
        long long n = getNum(num.size());
        while (--n) {
            nextPermutation(num);
            res.push_back(num);
        }
        return res;
    }
};