[FIXED] Is there a way to concatenate Java strings in less than O(n) time?

Issue

My homework question involves joining strings in a particular sequence. We are first given the strings, followed by a set of instructions that tell us how to concatenate them; finally we print the output string.

I have used the Kattis FastIO class to handle buffered input and output. Below is my algorithm, which iterates through the instructions to concatenate the strings. I have tried making the array of normal strings, StringBuffers and StringBuilders.

The program seems to work as intended, but it gives a time limit error on my submission platform due to inefficiency. It seems like appending the way I did is O(n); is there any faster way?

public class JoinStrings {
    public static void main(String[] args) {
        Kattio io = new Kattio(System.in, System.out);
        ArrayList<StringBuilder> stringList = new ArrayList<StringBuilder>();
        int numStrings = io.getInt();
        StringBuilder[] stringArray = new StringBuilder[numStrings];

        for (int i = 0; i < numStrings; i++) {
            String str = io.getWord();
            stringArray[i] = new StringBuilder(str);
        }

        StringBuilder toPrint = stringArray[0]; 

        while (io.hasMoreTokens()) {
            int a = io.getInt();
            int b = io.getInt();
            stringArray[a-1].append(stringArray[b-1]); // this is the line that is done N times

            toPrint = stringArray[a-1];
        }

        io.println(toPrint.toString());
        io.flush();
    }
} 

Solution

The StringBuilder.append() copy char from new string to existing string. It's fast but not free.

Instead of keeping appending the String to the StringBuilder array, keep track of the String indexes need to appended. Then finally append the Strings stored in the print out indexes list.

Answered By - Mạnh Quyết Nguyễn
Answer Checked By - Terry (JavaFixing Volunteer)