PermMissingElem

An array A consisting of N different integers is given. The array contains integers in the range [1..(N + 1)], which means that exactly one element is missing.

Your goal is to find that missing element.

Write a function:

class Solution { public int solution(int[] A); }

that, given an array A, returns the value of the missing element.

For example, given array A such that:

  A[0] = 2
  A[1] = 3
  A[2] = 1
  A[3] = 5

the function should return 4, as it is the missing element.

Assume that:

N is an integer within the range [0..100,000]; the elements of A are all distinct; each element of array A is an integer within the range [1..(N + 1)].

Complexity:

expected worst-case time complexity is O(N); expected worst-case space complexity is O(1) (not counting the storage required for input arguments).

Solution:

class Solution {
    public int solution(int[] A) {
        // write your code in Java SE 8
        int length = A.length;
        long sum = (length+1)*(length+2)/2;
        for(int i = 0; i < length; i++) {
            sum -= A[i];
        }
        return (int)sum;
    }
}