solves buddy strings

Author: anishLearnsToCode

Diff for: README.md

@@ -230,7 +230,7 @@
 | 844 | [Backspace String Compare](https://leetcode.com/problems/backspace-string-compare) | [![Java](assets/java.png)](src/BackspaceStringCompare.java) |
 | 849 | [Maximize Distance to Closest Person](https://leetcode.com/problems/maximize-distance-to-closest-person) | |
 | 852 | [Peak Index in Mountain Array](https://leetcode.com/problems/peak-index-in-a-mountain-array) | [![Java](assets/java.png)](src/PeakIndexInMountainArray.java) |
-| 859 | [Buddy Strings](https://leetcode.com/problems/buddy-strings) | |
+| 859 | [Buddy Strings](https://leetcode.com/problems/buddy-strings) | [![Java](assets/java.png)](src/BuddyStrings.java) |
 | 860 | [Lemonade Change](https://leetcode.com/problems/lemonade-change) | |
 | 867 | [Transpose Matrix](https://leetcode.com/problems/transpose-matrix) | |
 | 868 | [Binary Gap](https://leetcode.com/problems/binary-gap) | |

Diff for: src/BuddyStrings.java

@@ -0,0 +1,30 @@
+import java.util.Arrays;
+
+public class BuddyStrings {
+    public boolean buddyStrings(String s, String goal) {
+        if (s.length() != goal.length()) return false;
+        final int[] characterFrequencies = new int[26];
+        final int[] need = new int[26];
+        int differences = 0;
+        char c;
+        for (int index = 0 ; index < s.length() ; index++) {
+            c = s.charAt(index);
+            characterFrequencies[c - 'a']++;
+            if (c != goal.charAt(index)) {
+                differences++;
+                need[goal.charAt(index) - 'a']++;
+                need[c - 'a']--;
+            }
+        }
+        if (differences == 1 || differences > 2) return false;
+        if (differences == 0 && oneCharOccurrsMoreThanOnce(characterFrequencies)) return true;
+        return differences == 2 && Arrays.stream(need).allMatch(val -> val == 0);
+    }
+
+    private boolean oneCharOccurrsMoreThanOnce(int[] frequencies) {
+        for (int frequency : frequencies) {
+            if (frequency > 1) return true;
+        }
+        return false;
+    }
+}