# 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”.

``````class Solution {
public:
vector<string> res;
for (int h = 0; h < 12; ++h) {
for (int m = 0; m < 60; ++m) {
if (bitset<10>((h << 6) + m).count() == num) {
res.push_back(to_string(h) + (m < 10 ? ":0" : ":") + to_string(m));
}
}
}
return res;
}
};
``````

``````class Solution {
public:
vector<string> res;
vector<int> hour{8, 4, 2, 1}, minute{32, 16, 8, 4, 2, 1};
for (int i = 0; i <= num; ++i) {
vector<int> hours = generate(hour, i);
vector<int> minutes = generate(minute, num - i);
for (int h : hours) {
if (h > 11) continue;
for (int m : minutes) {
if (m > 59) continue;
res.push_back(to_string(h) + (m < 10 ? ":0" : ":") + to_string(m));
}
}
}
return res;
}
vector<int> generate(vector<int>& nums, int cnt) {
vector<int> res;
helper(nums, cnt, 0, 0, res);
return res;
}
void helper(vector<int>& nums, int cnt, int pos, int out, vector<int>& res) {
if (cnt == 0) {
res.push_back(out);
return;
}
for (int i = pos; i < nums.size(); ++i) {
helper(nums, cnt - 1, i + 1, out + nums[i], res);
}
}
};
``````

``````class Solution {
public:
vector<vector<int>> hours{{0},{1,2,4,8},{3,5,9,6,10},{7,11}};
vector<vector<int>> minutes{{0},{1,2,4,8,16,32},{3,5,9,17,33,6,10,18,34,12,20,36,24,40,48},{7,11,19,35,13,21,37,25,41,49,14,22,38,26,42,50,28,44,52,56},{15,23,39,27,43,51,29,45,53,57,30,46,54,58},{31,47,55,59}};
vector<string> res;
for (int k = 0; k <= num; ++k) {
int t = num - k;
if (k > 3 || t > 5) continue;
for (int i = 0; i < hours[k].size(); ++i) {
for (int j = 0; j < minutes[t].size(); ++j) {
string str = minutes[t][j] < 10 ? "0" + to_string(minutes[t][j]) : to_string(minutes[t][j]);
res.push_back(to_string(hours[k][i]) + ":" + str);
}
}
}
return res;
}
};
``````

https://discuss.leetcode.com/topic/59374/simple-python-java

https://discuss.leetcode.com/topic/59401/straight-forward-6-line-c-solution-no-need-to-explain

https://discuss.leetcode.com/topic/59494/3ms-java-solution-using-backtracking-and-idea-of-permutation-and-combination/2

LeetCode All in One 题目讲解汇总(持续更新中…)

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