Algorithm1.java

/**
 *
 * @author Kareem Khattab
 */

/**
 *Given a string, return this output 
 *“3[a]4[b]” → aaabbbb 
 *“5[c3[b]]” → cbbbcbbbcbbbcbbbcbbb 
 *“2[z]3[z2[a]]” → zzzaazaazaa 
 */

public class Algorithm1 {

    public static void main(String[] args) {

        String pattern = "2[3[a]3[b]3[c]]";

        System.out.println("Extracting pattern: " + pattern);
        System.out.println(getOutput(pattern));
    }

    private static String getOutput(String pattern) {
        
        int depth = 0;
        int start = 0;
        int repeat = 0;
        char current;

        //buffer so I can append my output then use .toString() on the buffer
        // Strings are immutable types in Java
        StringBuffer output = new StringBuffer();

        for (int i = 0; i < pattern.length(); i++) {
            current = pattern.charAt(i);
            if ('0' <= current && '9' >= current) {
                if (depth == 0) {
                    repeat = repeat * 10 + (current - '0');
                }
            } else if ('[' == current) {
                if (depth == 0) {
                    start = i + 1;
                }
                depth += 1;
            } else if (']' == current) {
                depth -= 1;
                if (depth == 0) {
                    String result = getOutput(pattern.substring(start, i));

                    for (; repeat > 0; repeat--) {
                        output.append(result);
                    }
                    repeat = 0;
                    start = 0;
                }
            } else
            if (depth == 0) {
                output.append(current);
            }
        }
        return output.toString();
    }
}