class Solution {
public:
    void nextPermutation(vector<int>& num) {

       // next_permutation(nums.begin(), nums.end());

        if (num.size() < 2) return;

        // in reverse order, find the first number which is in increasing trend (we call it violated number here)
        int i;
        for (i = num.size()-2; i >= 0; --i)
        {
            if (num[i] < num[i+1]) break;
        }

        // reverse all the numbers after violated number
        reverse(begin(num)+i+1, end(num));
        // if violated number not found, because we have reversed the whole array, then we are done!
        if (i == -1) return;
        // else binary search find the first number larger than the violated number
        auto itr = upper_bound(begin(num)+i+1, end(num), num[i]);

        swap(num[i], *itr);
    }
};