binary_watch.dart

/*

-* 401. Binary Watch *-

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


Given an integer turnedOn which represents the number of LEDs that are currently on (ignoring the PM), 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

*/

import 'dart:collection';
import 'dart:core';

class Solution {
  List<String> readBinaryWatch(int turnedOn) {
    List<int> dp = List.filled(60, 0); // number of set bits in ith
    for (int i = 1; i < 60; i++) {
      dp[i] = dp[i ~/ 2] + (i & 1);
    }

    List<String> res = [];

    for (int hr = 0; hr < 12; hr++) {
      for (int i = 0; i < 60; i++) {
        if (dp[i] + dp[hr] == turnedOn) {
          if (i < 10) {
            res.add(hr.toString() + ":0" + i.toString());
          } else {
            res.add(hr.toString() + ":" + i.toString());
          }
        }
      }
    }
    return res;
  }
}

class B {
  List<String> readBinaryWatch(int turnedOn) {
    List<String> result = [];
    List<int> used = List.filled(10, 0);
    helper(turnedOn, used, result, 0);
    //  Collections.sort(result);
    result.sort();
    return result;
  }

  void helper(int num, List<int> used, List<String> result, int start) {
    if (num == 0) {
      String? time = getTime(used);
      if (time != null) result.add(time);
      return;
    }
    for (int i = start; i < used.length; i++) {
      if (used[i] == 0) {
        used[i] = 1;
        helper(num - 1, used, result, i + 1);
        used[i] = 0;
      }
    }
  }

  String? getTime(List<int> used) {
    HashMap<int, int> idxToVal = HashMap();
    idxToVal.addAll({0: 1, 1: 2, 2: 4, 4: 1, 5: 2, 6: 4, 7: 8, 8: 16, 9: 32});
    int hour = 0, mins = 0;
    for (int i = 0; i < used.length; i++) {
      if (used[i] == 1) {
        if (i <= 3) {
          hour += idxToVal[i] ?? 0;
        } else {
          mins += idxToVal[i] ?? 0;
        }
      }
    }
    if (hour >= 12 || mins >= 60) return null;

    StringBuffer timeStr = StringBuffer(hour.toString() + ":");
    if (mins <= 9) timeStr.write("0");
    timeStr.write(mins);
    return timeStr.toString();
  }
}

class C {
  List<String> readBinaryWatch(int num) {
    List<String> result = [];

    //range 0-3 are hours and range 4-9 are minutes
    List<int> arr = [1, 2, 4, 8, 1, 2, 4, 8, 16, 32];
    backtrack(arr, 0, 0, 0, num, result);
    return result;
  }

  void backtrack(List<int> arr, int position, int hours, int minutes, int limit,
      List<String> result) {
    //when num(limit) reaches to the zero, store hours and minutes into result list
    if (limit == 0) {
      //during recursion we might get e.g 4 + 8 = 12 hours which we must skip because max hour value could be 11
      if (hours <= 11 && minutes <= 59) {
        StringBuffer builder = StringBuffer();
        builder.write(hours);
        builder.write(":");
        builder.write(minutes <= 9 ? "0" + minutes.toString() : minutes);
        result.add(builder.toString());
      }
      return;
    }

    //standard backtracking solution add new value do recursion and then remove it
    for (int i = position; i < arr.length; i++) {
      if (isHour(i))
        hours += arr[i];
      else
        minutes += arr[i];

      backtrack(arr, i + 1, hours, minutes, limit - 1, result);

      if (isHour(i))
        hours -= arr[i];
      else
        minutes -= arr[i];
    }
  }

  //Simple check by range: hours or minutes
  bool isHour(int position) {
    return position >= 0 && position <= 3;
  }
}