add 401

README.md

@@ -287,6 +287,7 @@ My LeetCode solutions with Chinese explanation. 我的LeetCode中文题解。
 | 397 |[Integer Replacement](https://leetcode.com/problems/integer-replacement/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/397.%20Integer%20Replacement.md)|Medium|  |
 | 398 |[Random Pick Index](https://leetcode.com/problems/random-pick-index/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/398.%20Random%20Pick%20Index.md)|Medium|  |
 | 400 |[Nth Digit](https://leetcode.com/problems/nth-digit)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/400.%20Nth%20Digit.md)|Medium|  |
+| 401 |[Binary Watch](https://leetcode.cn/problems/binary-watch/)|[C++](solutions/401.%20Binary%20Watch.md)|Easy|  |
 | 402 |[Remove K Digits](https://leetcode.cn/problems/remove-k-digits/)|[C++](solutions/402.%20Remove%20K%20Digits.md)|Medium|  |
 | 404 |[Sum of Left Leaves](https://leetcode.com/problems/sum-of-left-leaves)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/404.%20Sum%20of%20Left%20Leaves.md)|Easy|  |
 | 405 |[Convert a Number to Hexadecimal](https://leetcode.com/problems/convert-a-number-to-hexadecimal)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/405.%20Convert%20a%20Number%20to%20Hexadecimal.md)|Easy|  |

solutions/401. Binary Watch.md

@@ -0,0 +1,33 @@
+# [401. Binary Watch](https://leetcode.cn/problems/binary-watch/)
+
+# 思路
+由于分钟和小时都是有限且很小的，所以我们可以遍历一遍所有分钟，检查其二进制表示中1的个数即可。是个简单题，不要考虑太复杂。
+
+# C++
+```C++
+class Solution {
+private:
+    int bitCount(int num){
+        int res = 0;
+        while(num > 0){
+            res += (num % 2);
+            num /= 2;
+        }
+        return res;
+    }
+public:
+    vector<string> readBinaryWatch(int turnedOn) {
+        vector<string>res;
+        for(int i = 0; i < 12; i++){
+            for(int j = 0; j < 60; j++){
+                if(bitCount(i) + bitCount(j) == turnedOn){
+                    res.push_back(
+                        to_string(i) + (j < 10 ? ":0" : ":") + to_string(j)
+                    );
+                }
+            }
+        }
+        return res;
+    }
+};
+```