QuestionB.java

package Q17_05_Letters_and_Numbers;

import java.util.HashMap;


public class QuestionB {
	/* Compute the difference between the number of letters and 
	 * numbers between the beginning of the array and each index. */
	public static int[] computeDeltaArray(char[] array) {
		int[] deltas = new int[array.length];
		int delta = 0;
		for (int i = 0; i < array.length; i++) {
			if (Character.isLetter(array[i])) {
				delta++;
			} else if (Character.isDigit(array[i])) {
				delta--;
			}
			deltas[i] = delta;
		}
		return deltas;
	}
	
	/* Find the matching pair of values in the deltas array with the 
	 * largest difference in indices. */ 	
	public static int[] findLongestMatch(int[] deltas) {
		HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
		map.put(0,  -1);
		int[] max = new int[2];
		for (int i = 0; i < deltas.length; i++) {
			if (!map.containsKey(deltas[i])) {
				map.put(deltas[i],  i);
			} else {
				int match = map.get(deltas[i]);
				int distance = i - match;
				int longest = max[1] - max[0];
				if (distance > longest) {
					max[1] = i;
					max[0] = match;
				}
			}
		}
		return max;
	}
	
	public static char[] extract(char[] array, int start, int end) {
		if (start > end) return null;
		char[] subarray = new char[end - start + 1];
		for (int i = start; i <= end; i++) {
			subarray[i - start] = array[i];
		}
		return subarray;
	}

	public static char[] findLongestSubarray(char[] array) {
		/* Compute deltas betw count of numbers and count of letters. */
		int[] deltas = computeDeltaArray(array);

		/* Find pair in deltas with matching values and largest span. */
		int[] match = findLongestMatch(deltas);

		/* Return the subarray. Note that it starts one *after* the 
		 * initial occurence of this delta. */
		return extract(array, match[0] + 1, match[1]);
	}
	
	public static boolean isEqual(char[] array, int start, int end) {
		int counter = 0;
		for (int i = start; i < end; i++) {
			if (Character.isLetter(array[i])) {
				counter++;
			} else if (Character.isDigit(array[i])) {
				counter--;
			}
		}
		return counter == 0;
	}
	
	public static void main(String[] args) {
		char b = '1';
		char a = 'a';
		char[] array = {a, b, a, b, a, b, b, b, b, b, a, a, a, a, a, b, a, b, a, b, b, a, a, a, a, a, a, a};
		for (int i = 0; i < array.length; i++) {
			System.out.print(array[i] + " ");
		}
		System.out.println();
		char[] max = findLongestSubarray(array);
		if (max == null) {
			System.out.println("No equal subarray");
		} else {
			System.out.println(max.length);
			for (int i = 0; i < max.length; i++) {
				System.out.print(max[i] + " ");
			}
	
			System.out.println("\nIs Valid? " + isEqual(max, 0, max.length));
		}
	}

}