RLEIterator.java

//Write an iterator that iterates through a run-length encoded sequence.
//
//        The iterator is initialized by RLEIterator(int[] A), where A is a run-length encoding of some sequence.  More specifically, for all even i, A[i] tells us the number of times that the non-negative integer value A[i+1] is repeated in the sequence.
//
//        The iterator supports one function: next(int n), which exhausts the next n elements (n >= 1) and returns the last element exhausted in this way.  If there is no element left to exhaust, next returns -1 instead.
//
//        For example, we start with A = [3,8,0,9,2,5], which is a run-length encoding of the sequence [8,8,8,5,5].  This is because the sequence can be read as "three eights, zero nines, two fives".
//Example:
//        Input: ["RLEIterator","next","next","next","next"], [[[3,8,0,9,2,5]],[2],[1],[1],[2]]
//        Output: [null,8,8,5,-1]
//        Explanation:
//        RLEIterator is initialized with RLEIterator([3,8,0,9,2,5]).
//        This maps to the sequence [8,8,8,5,5].
//        RLEIterator.next is then called 4 times:
//
//        .next(2) exhausts 2 terms of the sequence, returning 8.  The remaining sequence is now [8, 5, 5].
//
//        .next(1) exhausts 1 term of the sequence, returning 8.  The remaining sequence is now [5, 5].
//
//        .next(1) exhausts 1 term of the sequence, returning 5.  The remaining sequence is now [5].
//
//        .next(2) exhausts 2 terms, returning -1.  This is because the first term exhausted was 5,
//        but the second term did not exist.  Since the last term exhausted does not exist, we return -1.
public class RLEIterator {
    int[] arr;
    int index = 0;

    public RLEIterator(int[] a) {
        arr = a;
    }

    public int next(int n) {
        int ans = -1;
        for (int i = index; i < arr.length - 1; i = i + 2) {
            if (arr[i] >= n) {
                ans = arr[i + 1];
                arr[i] = arr[i] - n;
                if (arr[i] == 0) {
                    index = i + 2;
                } else {
                    index = i;
                }
                break;

            } else {
                n = n - arr[i];
                arr[i] = 0;
            }
        }

        return ans;
    }
}