PermMissingElem 100% Solution in Javascript

Question

app.codility.com

My Solution

app.codility.com

function solution(A) {
    
    if (A.length === 0)
        return 1;
    
    let arrSum = A.reduce( (prev, curr) => prev + curr );
    let permSum = (1 + A.length + 1)*(A.length + 1)/2;
    
    return permSum-arrSum;    

}

Note

• Array sum using Array.reduce
• Beware of N=0 case!!