알고리즘 MissingInteger

Task description
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].

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    const sortArr = [...new Set(A.filter(el => el > 0).sort((a, b) => a-b))]
    if (sortArr.length === 0) return 1
    
    let result = sortArr[sortArr.length - 1] + 1
    for (let i=0; i<sortArr.length; i++) {
        if (sortArr[i] !== i + 1) {
            result = i + 1
            break
        }
    }
    
    return result
}