2 phone Interviews and 1 face to face interview. Interview questions. What is recall is this Quickest way to find missing number and missing slot in array of 100 elements Iterate through the array and compute the sum of all numbers. Now, sum of natural numbers from 1 to N, can be expressed as Nx(N+1)/2. N=100. Subtract the sum of the array from Nx(N+1)/2, where N=100. That is the missing number. The empty slot can be detected during the iteration in which the sum is computed. // will be the sum of the numbers in the array. int sum = 0; int idx = -1; for (int i = 0; i < arr.length; i++) { if (arr[i] == 0) { idx = i; } else { sum += arr[i]; } } // the total sum of numbers between 1 and arr.length. int total = (arr.length + 1) * arr.length / 2; System.out.println("missing number is: " + (total - sum) + " at index " + idx);