A recursive algorithm to find two integers in an array that sums to a given integer

Question

I need an algorithm to determine if an array contains two elements that sum to a given integer.

The array is sorted.

The algorithm should be recursiv

Answer 1

It is pretty easy. It is important for array to be sorted.

Correct algorithm with O(n) time complexity and no additional space is:

public static boolean isContainsSum(int[] arr, int sum) {
    for (int i = 0, j = arr.length - 1; i < j; ) {
        if (arr[i] + arr[j] == sum)
            return true;
        if (arr[i] + arr[j] < sum)
            i++;
        else
            j--;
    }

    return false;
}

To make it recursive, you need just replace i and j iterations with recursive call:

public static boolean isContainsSumRecursive(int[] arr, int sum) {
    return isContainsSumRecursive(arr, sum, 0, arr.length - 1);
}

private static boolean isContainsSumRecursive(int[] arr, int sum, int i, int j) {
    if (i == j)
        return false;
    if (arr[i] + arr[j] == sum)
        return true;
    if (arr[i] + arr[j] < sum)
        return isContainsSumRecursive(arr, sum, i + 1, j);
    return isContainsSumRecursive(arr, sum, i, j - 1);
}