401. Binary Watch

Description

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 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

Solutions

Solution 1

Python3JavaC++GoTypeScriptRust

class Solution:
    def readBinaryWatch(self, turnedOn: int) -> List[str]:
        return [
            '{:d}:{:02d}'.format(i, j)
            for i in range(12)
            for j in range(60)
            if (bin(i) + bin(j)).count('1') == turnedOn
        ]

class Solution {
    public List<String> readBinaryWatch(int turnedOn) {
        List<String> ans = new ArrayList<>();
        for (int i = 0; i < 12; ++i) {
            for (int j = 0; j < 60; ++j) {
                if (Integer.bitCount(i) + Integer.bitCount(j) == turnedOn) {
                    ans.add(String.format("%d:%02d", i, j));
                }
            }
        }
        return ans;
    }
}

class Solution {
public:
    vector<string> readBinaryWatch(int turnedOn) {
        vector<string> ans;
        for (int i = 0; i < 12; ++i) {
            for (int j = 0; j < 60; ++j) {
                if (__builtin_popcount(i) + __builtin_popcount(j) == turnedOn) {
                    ans.push_back(to_string(i) + ":" + (j < 10 ? "0" : "") + to_string(j));
                }
            }
        }
        return ans;
    }
};

func readBinaryWatch(turnedOn int) []string {
    var ans []string
    for i := 0; i < 12; i++ {
        for j := 0; j < 60; j++ {
            if bits.OnesCount(uint(i))+bits.OnesCount(uint(j)) == turnedOn {
                ans = append(ans, fmt.Sprintf("%d:%02d", i, j))
            }
        }
    }
    return ans
}

function readBinaryWatch(turnedOn: number): string[] {
    if (turnedOn === 0) {
        return ['0:00'];
    }
    const n = 10;
    const res = [];
    const bitArr = new Array(10).fill(false);
    const createTime = () => {
        return [
            bitArr.slice(0, 4).reduce((p, v) => (p << 1) | Number(v), 0),
            bitArr.slice(4).reduce((p, v) => (p << 1) | Number(v), 0),
        ];
    };
    const helper = (i: number, count: number) => {
        if (i + count > n || count === 0) {
            return;
        }
        bitArr[i] = true;
        if (count === 1) {
            const [h, m] = createTime();
            if (h < 12 && m < 60) {
                res.push(`${h}:${m < 10 ? '0' + m : m}`);
            }
        }
        helper(i + 1, count - 1);
        bitArr[i] = false;
        helper(i + 1, count);
    };
    helper(0, turnedOn);
    return res;
}

impl Solution {
    fn create_time(bit_arr: &[bool; 10]) -> (i32, i32) {
        let mut h = 0;
        let mut m = 0;
        for i in 0..4 {
            h <<= 1;
            h |= if bit_arr[i] { 1 } else { 0 };
        }
        for i in 4..10 {
            m <<= 1;
            m |= if bit_arr[i] { 1 } else { 0 };
        }

        (h, m)
    }

    fn helper(res: &mut Vec<String>, bit_arr: &mut [bool; 10], i: usize, count: usize) {
        if i + count > 10 || count == 0 {
            return;
        }
        bit_arr[i] = true;
        if count == 1 {
            let (h, m) = Self::create_time(bit_arr);
            if h < 12 && m < 60 {
                if m < 10 {
                    res.push(format!("{}:0{}", h, m));
                } else {
                    res.push(format!("{}:{}", h, m));
                }
            }
        }
        Self::helper(res, bit_arr, i + 1, count - 1);
        bit_arr[i] = false;
        Self::helper(res, bit_arr, i + 1, count);
    }

    pub fn read_binary_watch(turned_on: i32) -> Vec<String> {
        if turned_on == 0 {
            return vec![String::from("0:00")];
        }
        let mut res = vec![];
        let mut bit_arr = [false; 10];
        Self::helper(&mut res, &mut bit_arr, 0, turned_on as usize);
        res
    }
}

Solution 2

Python3JavaC++Go

class Solution:
    def readBinaryWatch(self, turnedOn: int) -> List[str]:
        ans = []
        for i in range(1 << 10):
            h, m = i >> 6, i & 0b111111
            if h < 12 and m < 60 and i.bit_count() == turnedOn:
                ans.append('{:d}:{:02d}'.format(h, m))
        return ans

class Solution {
    public List<String> readBinaryWatch(int turnedOn) {
        List<String> ans = new ArrayList<>();
        for (int i = 0; i < 1 << 10; ++i) {
            int h = i >> 6, m = i & 0b111111;
            if (h < 12 && m < 60 && Integer.bitCount(i) == turnedOn) {
                ans.add(String.format("%d:%02d", h, m));
            }
        }
        return ans;
    }
}

class Solution {
public:
    vector<string> readBinaryWatch(int turnedOn) {
        vector<string> ans;
        for (int i = 0; i < 1 << 10; ++i) {
            int h = i >> 6, m = i & 0b111111;
            if (h < 12 && m < 60 && __builtin_popcount(i) == turnedOn) {
                ans.push_back(to_string(h) + ":" + (m < 10 ? "0" : "") + to_string(m));
            }
        }
        return ans;
    }
};

func readBinaryWatch(turnedOn int) []string {
    var ans []string
    for i := 0; i < 1<<10; i++ {
        h, m := i>>6, i&0b111111
        if h < 12 && m < 60 && bits.OnesCount(uint(i)) == turnedOn {
            ans = append(ans, fmt.Sprintf("%d:%02d", h, m))
        }
    }
    return ans
}

Comments