401 Binary Watch 二进制手表

详见：https://leetcode.com/problems/binary-watch/description/

C++：

class Solution {
public:
    vector<string> readBinaryWatch(int num) 
    {
        vector<string> res;
        vector<int> hour{8, 4, 2, 1}, minute{32, 16, 8, 4, 2, 1};
        for (int i = 0; i <= num; ++i) 
        {
            vector<int> hours = generate(hour, i);
            vector<int> minutes = generate(minute, num - i);
            for (int h : hours)
            {
                if (h > 11)
                {
                    continue;
                }
                for (int m : minutes)
                {
                    if (m > 59)
                    {
                        continue;
                    }
                    res.push_back(to_string(h) + (m < 10 ? ":0" : ":") + to_string(m));
                }
            }
        }
        return res;
    }
    vector<int> generate(vector<int>& nums, int cnt) 
    {
        vector<int> res;
        helper(nums, cnt, 0, 0, res);
        return res;
    }
    void helper(vector<int>& nums, int cnt, int pos, int out, vector<int>& res) 
    {
        if (cnt == 0) 
        {
            res.push_back(out);
            return;
        }
        for (int i = pos; i < nums.size(); ++i) 
        {
            helper(nums, cnt - 1, i + 1, out + nums[i], res);
        }
    }
};

 参考：https://www.cnblogs.com/grandyang/p/5896454.html