solves rotate string

Author: anishLearnsToCode

README.md

@@ -214,7 +214,7 @@
 | 771 | [Jewels and Stones](https://leetcode.com/problems/jewels-and-stones) | [![Java](assets/java.png)](src/JewelsAndStones.java) [![Python](assets/python.png)](python/jewels_and_stones.py) |
 | 783 | [Minimum Distance Between BST Nodes](https://leetcode.com/problems/minimum-distance-between-bst-nodes) | [![Java](assets/java.png)](src/MinimumAbsoluteDifferenceInBST.java) [![Python](assets/python.png)](python/minimum_distance_between_bst_nodes.py) |
 | 788 | [Rotated Digits](https://leetcode.com/problems/rotated-digits) | |
-| 796 | [Rotate String](https://leetcode.com/problems/rotate-string) | |
+| 796 | [Rotate String](https://leetcode.com/problems/rotate-string) | [![Java](assets/java.png)](src/RotateString.java)  |
 | 800 | [Similar RGB Color](https://leetcode.com/problems/similar-rgb-color) | |
 | 804 | [Unique Morse Code Words](https://leetcode.com/problems/unique-morse-code-words) | |
 | 806 | [Number of Lines to Write String](https://leetcode.com/problems/number-of-lines-to-write-string) | |

src/RotateString.java

@@ -0,0 +1,41 @@
+public class RotateString {
+    public boolean rotateString(String s, String goal) {
+        if (s.length() != goal.length()) return false;
+        if (s.equals(goal) || s.length() == 0) return true;
+        return patternExists(s + s, goal);
+    }
+
+    private boolean patternExists(String string, String pattern) {
+        return kmpIndex(string, pattern) != -1;
+    }
+
+    private int kmpIndex(String string, String pattern) {
+        int[] patternPrefix = patternPrefixArray(pattern);
+        for (int t = 0, p = 0 ; t < string.length() && p < pattern.length() ; ) {
+            if (string.charAt(t) == pattern.charAt(p)) {
+                if (p == pattern.length() - 1) return t - p;
+                p++;
+                t++;
+            } else if (p != 0) {
+                p = patternPrefix[p - 1];
+            } else {
+                t++;
+            }
+        }
+        return -1;
+    }
+
+    private int[] patternPrefixArray(String pattern) {
+        int[] patternPrefix = new int[pattern.length()];
+        for (int j = 0, i = 1 ; i  < pattern.length() && j < pattern.length() ; ) {
+            if (pattern.charAt(j) == pattern.charAt(i)) {
+                patternPrefix[i++] = j++ + 1;
+            } else if (j == 0) {
+                patternPrefix[i++] = 0;
+            } else {
+                j = patternPrefix[j - 1];
+            }
+        }
+        return patternPrefix;
+    }
+}