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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0401_0500.s0401_binary_watch;

// #Easy #Bit_Manipulation #Backtracking #2022_07_15_Time_1_ms_(99.26%)_Space_43.1_MB_(42.54%)

import java.util.ArrayList;
import java.util.List;

/**
 * 401 - Binary Watch\.
 *
 * Easy
 *
 * 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 below binary watch reads `"4:51"`.
 *
 * ![](https://assets.leetcode.com/uploads/2021/04/08/binarywatch.jpg)
 *
 * Given an integer `turnedOn` which represents the number of LEDs that are currently on, return _all possible times the watch could represent_. You may return the answer in **any order**.
 *
 * 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"`.
 *
 * **Example 1:**
 *
 * **Input:** turnedOn = 1
 *
 * **Output:** ["0:01","0:02","0:04","0:08","0:16","0:32","1:00","2:00","4:00","8:00"] 
 *
 * **Example 2:**
 *
 * **Input:** turnedOn = 9
 *
 * **Output:** [] 
 *
 * **Constraints:**
 *
 * *   `0 <= turnedOn <= 10`
**/
public class Solution {
    public List readBinaryWatch(int turnedOn) {
        List times = new ArrayList<>();
        for (int hour = 0; hour <= 11; hour++) {
            for (int minutes = 0; minutes <= 59; minutes++) {
                readBinaryWatchHelper(turnedOn, times, hour, minutes);
            }
        }
        return times;
    }

    private void readBinaryWatchHelper(
            int turnedOn, List selectedTimes, int hour, int minutes) {
        if (isValidTime(turnedOn, hour, minutes)) {
            selectedTimes.add(getTimeString(hour, minutes));
        }
    }

    private String getTimeString(int hour, int minutes) {
        StringBuilder time = new StringBuilder();
        time.append(hour);
        time.append(':');
        if (minutes < 10) {
            time.append('0');
        }
        time.append(minutes);
        return time.toString();
    }

    private boolean isValidTime(int turnedOn, int hour, int minutes) {
        int counter = 0;
        while (hour != 0) {
            if ((hour & 1) == 1) {
                counter++;
            }
            hour >>>= 1;
        }
        if (counter > turnedOn) {
            return false;
        }
        while (minutes != 0) {
            if ((minutes & 1) == 1) {
                counter++;
            }
            minutes >>>= 1;
        }
        return counter == turnedOn;
    }
}