401. Binary Watch

A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).

Each LED represents a zero or one, with the least significant bit on the right.

For example, the above binary watch reads "3:25".

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.

Example:

Input: n = 1
Return: ["1:00", "2:00", "4:00", "8:00", "0:01", "0:02", "0:04", "0:08", "0:16", "0:32"]

 

Note:

• The order of output does not matter.
• The hour must not contain a leading zero, for example "01:00" is not valid, it should be "1:00".
• The minute must be consist of two digits and may contain a leading zero, for example "10:2" is not valid, it should be "10:02".

 

class Solution {
public:
    vector<string> readBinaryWatch(int num) {
        vector<int>hour = {1,2,4,8};
        vector<int>minute = {1,2,4,8,16,32};
        vector<string>res;
        pair<int,int>time = {0,0};
        dfs(0,time,num,hour,minute,res);
        return res;  
    }
    void dfs(int start,pair<int,int>time,int num,vector<int>hour,vector<int>minute,vector<string>&res)
    {
        if(num==0)
        {
            string result = to_string(time.first)+ (time.second<10?":0":":")+ to_string(time.second);
            res.push_back(result);
            return;
        }
        for(int i = start;i<10;i++)
        {
            if(i<4)
            {
                time.first+=hour[i];
                if(time.first<12) 
                    dfs(i+1,time,num-1,hour,minute,res);
                time.first-=hour[i];
            }
            else
            {
                time.second+=minute[i-4];
                if(time.second<60) 
                    dfs(i+1,time,num-1,hour,minute,res);
                time.second-=minute[i-4];
            }
        }
    }
};