Compress string: done

Author: rampatra

src/main/java/com/rampatra/strings/CompressString.java

@@ -0,0 +1,60 @@
+package com.rampatra.strings;
+
+/**
+ * @author rampatra
+ * @since 2019-04-08
+ */
+public class CompressString {
+
+    /**
+     * Compress a string, consisting of only alphabets, such that the compressed string contains the letter and
+     * a number next to it representing the number of times it is repeated. Also, compress only if the letter is
+     * repeated more than once and ignore it occurs just once.
+     * EXAMPLE:
+     * Input: aaabbcdd
+     * Output: a3b2cd2
+     * <p>
+     * Time Complexity: O(n)
+     * Space Complexity: O(n)
+     * where,
+     * n is the number of characters in the input string
+     *
+     * @param str the input string consisting on only alphabets
+     * @return the compressed string
+     */
+    private static String compress(String str) {
+        // some basic validation
+        if (str.length() == 0) {
+            throw new IllegalArgumentException("Empty String");
+        }
+        
+        StringBuilder sb = new StringBuilder();
+        int letterCount = 0;
+
+        for (int i = 0; i < str.length(); i++) {
+            /*
+                When the current character is a different one, append the previous character and its count to the
+                result, and finally, reset the counter
+             */
+            if (i != 0 && str.charAt(i) != str.charAt(i - 1)) {
+                sb.append(str.charAt(i - 1));
+                if (letterCount > 1) sb.append(letterCount);
+                letterCount = 0;
+            }
+            letterCount++;
+        }
+
+        // last character
+        sb.append(str.charAt(str.length() - 1));
+        if (letterCount > 1) sb.append(letterCount);
+
+        return sb.toString();
+    }
+
+    public static void main(String[] args) {
+        System.out.println(compress("a"));
+        System.out.println(compress("aabbcc"));
+        System.out.println(compress("aabcc"));
+        System.out.println(compress("aaaabbbccaad"));
+    }
+}