[lEET-401] add 401

fishercoder1534 · fishercoder1534 · commit e7615f1c3eb0 · 2017-02-19T12:16:37.000-08:00
diff --git a/leetcode-algorithms/README.md b/leetcode-algorithms/README.md
@@ -49,6 +49,7 @@
 |412|[Fizz Buzz](https://leetcode.com/problems/fizz-buzz/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/FizzBuzz.java)| O(n)|O(1) | Easy|
 |408|[Valid Word Abbreviation](https://leetcode.com/problems/valid-word-abbreviation/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/ValidWordAbbreviation.java)| O(n)|O(1) | Easy|
 |404|[Sum of Left Leaves](https://leetcode.com/problems/sum-of-left-leaves/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/SumofLeftLeaves.java)| O(n)|O(h) | Easy|
+|401|[Binary Watch](https://leetcode.com/problems/binary-watch/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/BinaryWatch.java)| O(1)|O(1) | Easy|
 |398|[Random Pick Index](https://leetcode.com/problems/random-pick-index/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/RandomPickIndex.java) | | | Medium| Reservoir Sampling
 |397|[Integer Replacement](https://leetcode.com/problems/integer-replacement/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/IntegerReplacement.java)| ? | ? | Easy| BFS
 |396|[Rotate Function](https://leetcode.com/problems/rotate-function/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/RotateFunction.java)| O(n^2) could be optimized to O(n) | O(1) | Easy|
diff --git a/leetcode-algorithms/src/test/java/com/stevesun/BinaryWatch.java b/leetcode-algorithms/src/test/java/com/stevesun/BinaryWatch.java
@@ -0,0 +1,37 @@
+package com.stevesun;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * 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".
+ */
+public class BinaryWatch {
+
+    public List<String> readBinaryWatch(int num) {
+        List<String> times = new ArrayList<>();
+        for (int h = 0; h < 12; h++) {
+            for (int m = 0; m < 60; m++) {
+                if (Integer.bitCount(h*60 + m) == num) times.add(String.format("%d:%02d", h, m));//%02 means to pad this two-digit decimal number on the left with zeroes
+            }
+        }
+        return times;
+    }
+
+}
