LeetCode – Refresh – Next Permutation

class Solution {
public:
    void swap(int &a, int &b){int t = a; a = b; b = t;};
    void reverse(vector<int> &num, int start, int end) {
        int len = (end - start + 1)/2;
        for (int i = 0; i < len; i++) swap(num[start+i], num[end-i]);
    }
    void nextPermutation(vector<int> &num) {
        int len = num.size();
        if (len < 2) return;
        for (int i = len-2; i >= 0; i--) {
            if (num[i] < num[i+1]) {
                for (int j = len-1; j > i; j--) {
                    if (num[i] < num[j]) {
                        swap(num[i], num[j]);
                        break;
                    }
                }
                reverse(num, i+1, len-1);
                return;
            }
        }
        reverse(num, 0, len-1);
    }
};