Given a hexadecimal string, s (e.g. "1a", "12b9c7"). Find out the minimum number of pieces, i, to split this string such that each of the i pieces is a hexadecimal representation of a perfect square.

PerfectRootHex.java

/*
Apollo likes perfect squares of integers (e.g. 1, 4, 9, 25 etc). You are given a hexadecimal string, s (e.g. "1a", "12b9c7"). Find out the minimum number of pieces, i, to split this string such that each of the i pieces is a hexadecimal representation of a perfect square. Note that this may not be possible for every hexadecimal string.

Complete the getMin function in your editor, in a language of your choice. It has one parameter: a string, s, a valid representation of a hexadecimal number. It must return an integer denoting the value of i; if there is no such positive integer i, return ?1.

Bonus: Performance is a concern. An unoptimized solution will see timeouts in some of the large test cases.

Input Format
The locked stub code in your editor reads a hexadecimal string, s, from stdin and passes it to your function. The input will conform to the following formats:

The characters in S are valid hexadecimal characters (0-9, a-f). You may assume the letters are in lowercase.

1 < |S| < 20

The hexadecimal string S does not start with "0x"

Sample values for S: "1a", "12b9c7", "a1111", "0001",

Output Format
Your function must return either some positive integer i (denoting the minimum number of pieces s must be cut into such that each piece corresponds with the hexadecimal representation of a perfect square), or ?1 if no such i exists.
Examples
Input: "896bb1"
Output: 1
Input: "0000000000000000000000000002"
Output: -1
 */
class PerfectRootHex {

    static char []chars = {'1', '4','9'};
  public static String Cus_free_apollo_f81d7Challenge(String str) {
  	int result = getMin(str, 0);
    if(result == Integer.MAX_VALUE)
    	return "-1";
    return String.valueOf(result);
  }

	public static int getMin(String hex, int index) {
  	if(index == hex.length()) 
    	return 0;
    int num = 0;

    int result = Integer.MAX_VALUE;
    for(int i=index; i<hex.length(); i++) {
        char ch = hex.charAt(i);
        num = num*16 + Integer.parseInt(String.valueOf(ch),16);
        if(ch == '1' || ch == '4' || ch == '9') {
            if (isPerfectSquare(num))
                result = Math.min(result, 1 + getMin(hex, i + 1));
        }
    }
    return result;
  }

    static boolean isPerfectSquare(int x) {
        if (x >= 0) {
            int sr = (int)Math.sqrt(x);
            return ((sr * sr) == x);
        }
        return false;
    }
    
    public static void main (String[] args) {
        System.out.println(Cus_free_apollo_f81d7Challenge("896bb1"));
        System.out.println(Cus_free_apollo_f81d7Challenge("0000000000000000000000000002"));
  }

}