My solution of the test task for a programmer position.
Task: An integer X and a non-empty zero-indexed array A consisting of N integers are given. We are interested in which elements of A are equal to X and which are different from X. The goal is to split array A into two parts, such that the number of elements equal to X in the first part is the same as the number of elements different from X in the other part. More formally, we are looking for an index K such that: 0 ≤ K < N and the number of elements equal to X in A[0..K−1] is the same as the number of elements different from X in A[K..N−1]. For K = 0, A[0..K−1] does not contain any elements.
For example, given integer X = 5 and array A such that:
A = [5, 5, 1, 7, 2, 3, 5]
K equals 4, because: two of the elements of A[0..3] are equal to X, namely A[0] = A[1] = X, and two of the elements of A[4..6] are different from X, namely A[4] and A[5].
Write a function solution($X, $A); that, given an integer X and a non-empty zero-indexed array A consisting of N integers, returns the value of index K satisfying the above conditions. If more than one index K satisfies the above conditions, your function may return any of them. If there is no such index, the function should return −1.
For example, given integer X and array A as above, the function should return 4, as explained above.
Assume that: N is an integer within the range [1..100,000]; X is an integer within the range [0..100,000]; Each element of array A is an integer within the range [0..100,000].
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.