401. Binary Watch

Description

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".

Solution

/**
 * @param {number} num
 * @return {string[]}
 */
var readBinaryWatch = function(num) {
    "use strict";
    if (num === 0) {
        return ["0:00"];
    }
    let result = [];
    let time = [1, 2, 4, 8, 1, 2, 4, 8, 16, 32];
    function possibleTimes(idx, depth, hour, min) {
        if (idx < 4) {
            if (hour+time[idx] > 11) {
                return;
            }
            hour += time[idx];
        } else {
            if (min+time[idx] > 59) {
                return;
            }
            min += time[idx];
        }
        if (depth === num) {
            if (min.toString().length === 1) {
                result.push(`${hour}:0${min}`);
            } else {
                result.push(`${hour}:${min}`);
            }
            return;
        }
        for (let i = idx+1; i < time.length; i++) {
            possibleTimes(i, depth+1, hour, min);
        }
    }
    for (let i = 0; i < time.length; i++) {
        possibleTimes(i, 1, 0, 0);
    }
    return result;
};