Missing Integer

Problem

This is a demo task.

Write a function:

function solution(A);

that, given an array A of N integers, returns the smallest positive integer (greater than 0) that does not occur in A.

For example, given A = [1, 3, 6, 4, 1, 2], the function should return 5.

Given A = [1, 2, 3], the function should return 4.

Given A = [−1, −3], the function should return 1.

Write an efficient algorithm for the following assumptions:

• N is an integer within the range [1..100,000];
• each element of array A is an integer within the range [−1,000,000..1,000,000].

My Solution

failed O(N**2)

function solution(A) {
	let maxVal = Math.max(...A);

	if (maxVal < 1) return 1;

	for (let i = 1; i < maxVal + 1; i++) {
		if (!A.includes(i)) return i;
	}

	return maxVal + 1;
}

failed O(N**2)

function solution(A) {
	A.sort((b, a) => a - b);
	let maxVal = A[0];

	if (maxVal < 1) return 1;

	for (let i = 1; i < maxVal + 1; i++) {
		if (!A.includes(i)) return i;
	}

	return maxVal + 1;
}

failed O(N**2)

function solution(A) {
	for (i = 1; i < 1000000; i++) {
		if (!A.includes(i)) return i;
	}
}

success O(N) or O(N * log(N))

function solution(A) {
	const set = new Set(A);
	let i = 1;
	while (set.has(i)) {
		i++;
	}
	return i;
}