L-03_Time-Complexity_PermMissingElem.js

// Find the missing element in a given permutation.
// Problem description: https://app.codility.com/programmers/lessons/3-time_complexity/perm_missing_elem/
// Detected time complexity: O(N) or O(N * log(N))

function solution(A) {
  const ALen = A.length;
  const maxNum = ALen + 1;
  // the sum of consecutive integers from 1 to n is equal to n(n+1) / 2.
  const totalSum = (maxNum * (maxNum + 1)) / 2;

  let partialSum = 0;
  for (let i = 0; i < ALen; i += 1) {
    partialSum += A[i];
  }
  return totalSum - partialSum;
}