gistfile1.txt

Example [1,2,4,6,3,7,8] Output: 5

We know Sum of first n natural numbers = n ( n+1) / 2

All the numbers are distinct starting from 1 and are in range 1 to n 

int findMissingNumber(int A[], int n) {
  int i = 0
  for (int j = 0; j<A.length ; j++){
    i = i + A[j]
   }
   int missing_no = n * (n+1) / 2 - i
   
   return missing_no
}