Calculating worst-case run time complexity of recursive algorithm

In clasas i have started learning how to calculate the run time complexity functions of various algorithms and am finding it difficult. I am trying to calculate the worst case run time complexity of my recursive algorithm below.

At the moment i am choosing my fundamental operation to be a comparison between the index of two chars, which occurs within an if-statement. However this if-statement is nested, and i am not sure how this affects t(n) within a recursive algorithm.

Would i be correct in thinking that the worst case run time complexity would be t(n) = N(N-1) = N^2 -1 or just O(n)=N^2? I got this logic from thinking that in a worst-case scenario each n chars would be checked in the outer if-statement, which would mean n-1 chars would be compared in the inner if statement.

public class StringShuffleTest {

    public static boolean isOrderedShuffle(String a, String b, String c){

        //variables for the size of Strings a, b and c.
        int n = a.length();
        int m = b.length();
        int len = c.length();     

        //if the length of c is not the length of a + b, return false.
        if (len != (n + m)){
            return false;
        }

        //if String c contains String b as a substring, then remove String b from c and make m = 0.
        //This statement avoids errors when dealing with Strings with very similar characters.
        if (c.contains(b)){
            c = c.replace(b, "");
            m = 0;
        }

        //if the length of a or b is 0, and c equals a or b, return true, otherwise,
        //return false.
        if (n == 0 || m == 0){
            if (c.equals(a) || c.equals(b)){
                return true;
            }
            else
                return false;
        }

        //if String a has length 1, remove a from String c and make String a empty.
        if (n == 1){
                c = c.substring(0, c.indexOf(a.charAt(0))) + c.substring(c.indexOf(a.charAt(0)) +1);
                a = "";
                return isOrderedShuffle(a, b, c);

            }

        //An ordered shuffle of two given strings, a and b, is a string that can be formed by interspersing
        //the characters of a and b in a way that maintains the left-to-right order of the characters from each
        //string.

        //Recursive algorithm to determine if String c is an ordered shuffle of a and b.
        else
        if (c.indexOf(a.charAt(0)) >= 0){

            int indexOfFirsta = c.indexOf(a.charAt(0));
            int indexOfSeconda = c.indexOf(a.charAt(1));

            if (indexOfFirsta <= indexOfSeconda){
            c = c.substring(0, indexOfFirsta) + c.substring(indexOfFirsta +1);
            a = a.substring(1, n);
                System.out.println(a);
                System.out.println(c);                   
            return isOrderedShuffle(a, b, c);
            }

        else
            if (c.indexOf(b.charAt(0)) >= 0){
                    int indexOfFirstb = c.indexOf(b.charAt(0));
                    int indexOfSecondb = c.indexOf(b.charAt(1));

                    if (indexOfFirstb <= indexOfSecondb){
                        c = c.substring(0, indexOfFirstb) + c.substring(indexOfFirstb +1);
                        b = b.substring(1, m);
                        System.out.println(b);
                        System.out.println(c);

                    return isOrderedShuffle(a, b, c);

                }
        }

        }
    return false;         
    }       

public static void main(String[] args) {

    System.out.println(StringShuffleTest.isOrderedShuffle("abc", "def", "abedcf")); 

}

}