Added palindrome problem asked in Delta X interview

Author: AdithyaBhat17

java/strings/HighestPalindrome.java

@@ -0,0 +1,90 @@
+/*
+Asked by - Delta X
+
+Make largest palindrome by changing at most K-digits
+Given a string containing all digits, we need to convert this string to a palindrome by changing at most K digits.
+If many solutions are possible then print lexicographically largest one.
+
+Examples:
+
+Input   : str = “43435”	
+          k = 3
+Output  : "93939" 
+Lexicographically largest palindrome 
+after 3 changes is "93939" 
+
+Input :  str = “43435”	
+         k = 1
+Output : “53435”
+Lexicographically largest palindrome 
+after 3 changes is “53435”
+
+Input  : str = “12345”	
+         k = 1
+Output : "Not Possible"
+It is not possible to make str palindrome
+after 1 change.
+*/
+
+/*
+if (minChanges == k) {
+    // if digits are different then change smaller to match larger
+} else if (minChanges >= k+2) {
+   // You have spare changes. Change one or both (or neither!) to '9'
+} else (minChanges == k+1) {
+   // You can make one extra change.  If the digits are different, then change the smaller
+   // to match the larger.
+}
+*/
+
+class Palindrome{
+    public static String maxPalindrome(String str, int k){
+        char[] c = str.toCharArray();
+        int minChange = 0;
+        for(int i=0,j=c.length-1;i<j;i++,j--)
+            if(c[i] != c[j])
+                minChange++;
+        
+        if(minChange > k)
+            return "-1";
+
+        int changeBoth = k - minChange;
+        int i = 0;
+        int j =c.length - 1;
+        for(;i<=j;i++,j--){
+            if(c[i] != c[j]){
+                char maxChar = (char) Math.max(c[i],c[j]);
+                if(maxChar != '9' && changeBoth - 1 >= 0){
+                    c[i] = '9';
+                    c[j] = '9';
+                    changeBoth--;
+                }
+                else{
+                    c[i] = maxChar;
+                    c[j] = maxChar;
+                    minChange--;
+                }
+            }
+            else{
+                char maxChar = (char) Math.max(c[i], c[j]);
+                if (maxChar != '9' && changeBoth - 2 >= 0) {
+                    c[i] = '9';
+                    c[j] = '9';
+                    changeBoth -= 2;
+                }
+            }
+        }
+        if (changeBoth != 0 && i - 1 == j + 1) {
+            c[i - 1] = '9';
+        }
+        String result = new String(c);
+        return result;
+    }
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        int n = in.nextInt();
+        int k = in.nextInt();
+        String str = in.next();
+        System.out.println(maxPalindrome(str, k));
+    }
+}