feat: added Lesson_13 solution for Chelsea

Author: Cogbonnia

lesson_13/maps_java/maps_app/src/main/java/com/codedifferently/lesson13/Lesson13.java

@@ -1,4 +1,6 @@
 package com.codedifferently.lesson13;
+import java.util.HashMap;
+import java.util.Map;
 
 public class Lesson13 {
 
@@ -7,6 +9,17 @@ public class Lesson13 {
    * https://leetcode.com/problems/permutation-difference-between-two-strings
    */
   public int findPermutationDifference(String s, String t) {
-    return 0;
+    Map<Character, Integer> indexMap = new HashMap<>();
+    for (int i = 0; i < s.length(); i++) {
+      indexMap.put(s.charAt(i), i);
+    }
+
+    int permutationDifference = 0;
+    for (int i = 0; i < t.length(); i++) {
+      char ch = t.charAt(i);
+      int indexInS = indexMap.get(ch);
+      permutationDifference += Math.abs(indexInS - i);
+    }
+    return permutationDifference;
   }
 }