Leetcode - 401. Binary Watch

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”.

1. Analyse

Do a loop of all time possibility and count the 1 bits of the binary expression.

2. AC Code

(32 Lines)

class Solution {
public:
    vector<string> readBinaryWatch(int num) {
        vector<string> ret;
        map<int, vector<int> > m;
        for( int i=0; i<60; i++ ){
            int count = 0, tmp = i;
            while( tmp ){  
                tmp = tmp & (tmp-1);
                count++;
            }   
            m[count].push_back( i );
        }   

        for( int i=0; i<=num; i++ ){
            vector<int> hour = m[i];
            for( int j=0; j<hour.size(); j++ ){
                int h = hour[j];
                if( h < 12 ){
                    vector<int> min = m[num-i];
                    for( int k=0; k<min.size(); k++ ){
                        if( min[k] <10 )
                            ret.push_back( to_string(h)+":0"+to_string(min[k]) );
                        else
                            ret.push_back( to_string(h)+":"+to_string(min[k]) );
                    }   
                }   
            }   
        }                           
        return ret;
    }   
};