Dominator.md

Task description

A zero-indexed array A consisting of N integers is given. The dominator of array A is the value that occurs in more than half of the elements of A.

For example, consider array A such that

 A[0] = 3    A[1] = 4    A[2] =  3
 A[3] = 2    A[4] = 3    A[5] = -1
 A[6] = 3    A[7] = 3

The dominator of A is 3 because it occurs in 5 out of 8 elements of A (namely in those with indices 0, 2, 4, 6 and 7) and 5 is more than a half of 8.

Write a function

function solution(A);

that, given a zero-indexed array A consisting of N integers, returns index of any element of array A in which the dominator of A occurs. The function should return −1 if array A does not have a dominator.

Assume that:

• N is an integer within the range [0..100,000];
• each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].

For example, given array A such that

 A[0] = 3    A[1] = 4    A[2] =  3
 A[3] = 2    A[4] = 3    A[5] = -1
 A[6] = 3    A[7] = 3

the function may return 0, 2, 4, 6 or 7, as explained above.

Complexity:

• expected worst-case time complexity is O(N);
• expected worst-case space complexity is O(1), beyond input storage (not counting the storage required for input arguments).

Elements of input arrays can be modified.

function solution(A) {
    if (A.length == 0) {
        return -1;
    }

    var pos = 0;
    var count = 0;

    for (var i = 0; i < A.length; i++) {
        if (A[pos] == A[i]) {
            count++;
        } else {
            count--;
            if (count == 0) {
                pos = i;
                count++;
            }
        }
    }

    count = 0;
    var cand = A[pos];
    
    for (var i in A) {
        if (A[i] == cand) {
            count++;
        }
    }

    if (count <= A.length / 2) {
        return -1;
    }

    return pos;
}