Shortest Palindrome

Author: gouthampradhan

README.md

@@ -280,6 +280,7 @@ My accepted leetcode solutions to some of the common interview problems.
 - [Longest Common Prefix](problems/src/string/LongestCommonPrefix.java) (Easy)
 - [Find the Closest Palindrome](problems/src/string/FindTheClosestPalindrome.java) (Hard)
 - [Monotone Increasing Digits](problems/src/string/MonotoneIncreasingDigits.java) (Medium)
+- [Shortest Palindrome](problems/src/string/ShortestPalindrome.java) (Hard)
 
 #### [Tree](problems/src/tree)
 

problems/src/string/ShortestPalindrome.java

@@ -0,0 +1,61 @@
+package string;
+
+/**
+ * Created by gouthamvidyapradhan on 21/07/2018.
+ *
+ * Given a string s, you are allowed to convert it to a palindrome by adding characters in front of it. Find and
+ * return the shortest palindrome you can find by performing this transformation.
+
+ Example 1:
+
+ Input: "aacecaaa"
+ Output: "aaacecaaa"
+ Example 2:
+
+ Input: "abcd"
+ Output: "dcbabcd"
+
+ Solution: O(N ^ 2): for i : (s.length() - 1 -> 0) check if (0, i) is a paliandrome, if not append char at i to
+ result string else return string (result + (0, i))
+ */
+public class ShortestPalindrome {
+
+    /**
+     * Main method
+     * @param args
+     * @throws Exception
+     */
+    public static void main(String[] args) throws Exception{
+        System.out.println(new ShortestPalindrome().shortestPalindrome("aaaaaaaaaa"));
+    }
+
+    public String shortestPalindrome(String s) {
+        if(s.length() == 0 || s.length() == 1){
+            return s;
+        } else if(s.length() == 2){
+            if(s.charAt(0) == s.charAt(1)){
+                return s;
+            } else{
+                return (s.charAt(1) + s);
+            }
+        }
+        if(isPaliandrome(s, 0, s.length() - 1)) return s;
+        StringBuilder sb = new StringBuilder("");
+        for(int i = 0, j = s.length() - 1; j >= i; j--){
+            if(!isPaliandrome(s, i, j)){
+                sb.append(s.charAt(j));
+            } else{
+                sb.append(s.substring(0, s.length()));
+                break;
+            }
+        }
+        return sb.toString();
+    }
+
+    boolean isPaliandrome(String s, int x, int y){
+        for(int i = x, j = y; i < j; i++, j--){
+            if(s.charAt(i) != s.charAt(j)) return false;
+        }
+        return true;
+    }
+}