refactor problem 347: add type for comparable

Author: ppulse

TopKFrequentElements347/Solution.java

@@ -1,15 +1,13 @@
 public class Solution {
-    class Pair implements Comparable {
+    class Pair implements Comparable<Pair> {
         public int key;
         public int value;
         public Pair(int k, int v) {
             key = k;
             value = v;
         }
 
-        public int compareTo(Object that) {
-            Pair other = (Pair)that;
-            
+        public int compareTo(Pair other) {
             if (value < other.value)
                 return 1;