Find the smallest positive integer that does not occur in a given sequence

100% result solution in Javascript:

function solution(A) {
    // only positive values, sorted
    A = A.filter(x => x >= 1).sort((a, b) => a - b)

    let x = 1

    for(let i = 0; i < A.length; i++) {
        // if we find a smaller number no need to continue, cause the array is sorted
        if(x < A[i]) {
            return x
        }
        x = A[i] + 1
    }

    return x
}

If the expected running time should be linear, you can't use a TreeSet, which sorts the input and therefore requires O(NlogN). Therefore you should use a HashSet, which requires O(N) time to add N elements.

Besides, you don't need 4 loops. It's sufficient to add all the positive input elements to a HashSet (first loop) and then find the first positive integer not in that Set (second loop).

int N = A.length;
Set<Integer> set = new HashSet<>();
for (int a : A) {
    if (a > 0) {
        set.add(a);
    }
}
for (int i = 1; i <= N + 1; i++) {
    if (!set.contains(i)) {
        return i;
    }
}

My code in Java, 100% result in Codility

import java.util.*;

class Solution {
    public int solution(int[] arr) {
        int smallestInt = 1;

        if (arr.length == 0) return smallestInt;

        Arrays.sort(arr);

        if (arr[0] > 1) return smallestInt;
        if (arr[arr.length - 1] <= 0) return smallestInt;

        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == smallestInt) {
                smallestInt++;
            }
        }

        return smallestInt;
    }
}

JS:

• filter to get positive non zero numbers from A array
• sort above filtered array in ascending order
• map to iterate loop of above stored result
  • if to check x is less than the current element then return
  • otherwise, add 1 in the current element and assign to x

function solution(A) {

    let x = 1
    
    A.filter(x => x >= 1)
     .sort((a, b) => a - b)
     .map((val, i, arr) => {
        if(x < arr[i]) return
        x = arr[i] + 1
    })

    return x
}

console.log(solution([3, 4, -1, 1]));
console.log(solution([1, 2, 0]));