add 1013

Author: fishercoder1534

README.md

@@ -27,6 +27,7 @@ Your ideas/fixes/algorithms are more than welcome!
 
 |  #  |      Title     |   Solutions   | Time          | Space         | Video  | Difficulty  | Tag                   
 |-----|----------------|---------------|---------------|---------------|--------|-------------|-------------
+|1013|[Pairs of Songs With Total Durations Divisible by 60](https://leetcode.com/problems/pairs-of-songs-with-total-durations-divisible-by-60/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1013.java) | O(n) | O(1) | |Easy|
 |1002|[Find Common Characters](https://leetcode.com/problems/find-common-characters/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1002.java) | O(n) | O(1) | |Easy|
 |999|[Available Captures for Rook](https://leetcode.com/problems/available-captures-for-rook/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_999.java) | O(1) | O(1) | |Easy|
 |997|[Find the Town Judge](https://leetcode.com/problems/find-the-town-judge/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_997.java) | O(n) | O(n) | |Easy| 

src/main/java/com/fishercoder/solutions/_1013.java

@@ -0,0 +1,53 @@
+package com.fishercoder.solutions;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * 1013. Pairs of Songs With Total Durations Divisible by 60
+ *
+ * In a list of songs, the i-th song has a duration of time[i] seconds.
+ *
+ * Return the number of pairs of songs for which their total duration in seconds is divisible by 60.
+ * Formally, we want the number of indices i < j with (time[i] + time[j]) % 60 == 0.
+ *
+ * Example 1:
+ * Input: [30,20,150,100,40]
+ * Output: 3
+ * Explanation: Three pairs have a total duration divisible by 60:
+ * (time[0] = 30, time[2] = 150): total duration 180
+ * (time[1] = 20, time[3] = 100): total duration 120
+ * (time[1] = 20, time[4] = 40): total duration 60
+ *
+ * Example 2:
+ * Input: [60,60,60]
+ * Output: 3
+ * Explanation: All three pairs have a total duration of 120, which is divisible by 60.
+ *
+ * Note:
+ *
+ * 1 <= time.length <= 60000
+ * 1 <= time[i] <= 500
+ * */
+public class _1013 {
+    public static class Solution1 {
+        /**Credit: https://leetcode.com/problems/pairs-of-songs-with-total-durations-divisible-by-60/discuss/256726/Java-O(n)-code-w-comment-similar-to-Two-Sum
+         *
+         * Think of Problem 1: Two Sum
+         * Assume target is 60, each item in time % 60.
+         * Then this problem becomes very similar to Problem 1.
+         * */
+        public int numPairsDivisibleBy60(int[] time) {
+            int result = 0;
+            Map<Integer, Integer> map = new HashMap<>();
+            for (int t : time) {
+                int d = (60 - t % 60) % 60;
+                if (map.containsKey(d)) {
+                    result += map.get(d);
+                }
+                map.put(t % 60, map.getOrDefault(t % 60, 0) + 1);
+            }
+            return result;
+        }
+    }
+}

src/test/java/com/fishercoder/_1013Test.java

@@ -0,0 +1,30 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._1013;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class _1013Test {
+    private static _1013.Solution1 solution1;
+    private static int[] time;
+
+    @BeforeClass
+    public static void setup() {
+        solution1 = new _1013.Solution1();
+    }
+
+    @Test
+    public void test1() {
+        time = new int[]{30, 20, 150, 100, 40};
+        assertEquals(3, solution1.numPairsDivisibleBy60(time));
+    }
+
+    @Test
+    public void test2() {
+        time = new int[]{60, 60, 60};
+        assertEquals(3, solution1.numPairsDivisibleBy60(time));
+    }
+
+}