Sandbox::https://github.com/prashanthigupta/leetcode-algorithms/blob/master/src/BinaryWatch.java

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import java.util.ArrayList;

import java.util.List;

public class BinaryWatch {

public List<String> readBinaryWatch(int num) {

List<String> result = new ArrayList<String>();

for(int i = 0 ; i < 12 ; i++){

for(int j=0 ; j < 60 ; j++){

int total = countDigits(i) + countDigits(j);

if(total == num){

StringBuilder time = new StringBuilder();

time.append(i)

.append(":")

.append(j < 10 ? 0 : "")

.append(j);

result.add(time.toString());

}

return result;

}

public int countDigits(int num){

int result = 0;

while(num > 0){

if((num & 1) == 1){

result++;

}

num >>= 1;

}

return result;

}