### 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".

### Analyze

• 前置条件: 确认如果一个 led 灯都没亮着是否为 "0:00"。
• 小时位数范围为 0, 11, 分钟范围为 0, 59
``````/**
* @param {number} num
* @return {string[]}
*/
const arr = [1, 2, 4, 8, 1, 2, 4, 8, 16, 32]
const result = []
backTrace(arr, num, 0, [0, 0], result)
return result
};

var backTrace = function(arr, num, start, temp, result) {
if (temp[0] >= 12 || temp[1] >= 60) return
if (num === 0) {
return
}

for (let i = start; i < arr.length; i++) {
if (i <= 3) {
temp[0] = temp[0] + arr[i]
} else {
temp[1] = temp[1] + arr[i]
}
num = num - 1
backTrace(arr, num, i + 1, temp, result)
if (i <= 3) {
temp[0] = temp[0] - arr[i]
} else {
temp[1] = temp[1] - arr[i]
}
num = num + 1
}
}