Find missing number in an array of numbers

scratch_16.js

// Find theoretical sum of the consecutive numbers using a variation of Gauss Sum.
// Formula: [(N * (N + 1)) / 2] - [(M * (M - 1)) / 2];
// N is the upper bound and M is the lower bound

function missingNumber(n) {
    let upperBound = 0;
    let lowerBound = 99999999;
    let sum = 0;

    for (let i = 0; i < n.length; i++) {
        sum += n[i];

        if(lowerBound > n[i]) lowerBound = n[i];
        if(n[i] > upperBound) upperBound = n[i];
    }

    const upperBoundLimitSum = (upperBound * (upperBound + 1)) / 2;
    const lowerBoundLimitSum = (lowerBound * (lowerBound - 1)) / 2;
    const theoreticalBound = upperBoundLimitSum - lowerBoundLimitSum

    return theoreticalBound - sum;
}

console.log(missingNumber([2, 5, 1, 4, 9, 6, 3, 7,10,11,12,13,14,15])) // 8

awli