Lesson3_1(PermMissingElem) - WhyAbout/CodilityAlgorithm GitHub Wiki

A zero-indexed 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:

int solution(vector<int> &A);

that, given a zero-indexed 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), beyond input storage (not counting the storage required for input arguments).
Elements of input arrays can be modified.

int solution(vector<int> &A) {
    // write your code in C++14 (g++ 6.2.0)
    unsigned int losted = A.size() + 1;
    for( unsigned int i = 0; i < A.size(); ++i) {
        losted = losted ^ A[i] ^ ( i + 1);
    }
    
    return losted;
}