find a position of an element occurrence in array

Question

I've below code snippet, A is non-decreasing order and need to check if X is present in A. If X is present in A then return position of occurrence of X in A otherwise returns -1. This code is not correct for some inputs, and need to fix it. Utmost I can modify 3 lines.

class Checking{
    int check(int[] A, int X) {
        int N = A.length;
        if (N == 0) {
            return -1;
        }
        int l = 0;
        int r = N - 1;
        while (l < r) {
            int m = (l + r) / 2;
            if (A[m] > X) {
                r = m - 1;
            } else {
                l = m;
            }
        }
        if (A[l] == X) {
            return l;
        }
        return -1;
    }
}

I couldnt figure out the fix, any suggestions will be helpful ?

Answer 1

Wiki page says that for such implementation of binary search middle element should be calculated using ceiling, so you can change to

int m = (l + r + 1) / 2;