title: Find The Missing Number 2
signature: function find_the_missing_number_2(a, n)
source_py: def find_the_missing_number_2(a, n):
      x1 = a[0]
      x2 = 1
      for i in range(1, n):
          x1 = (x1 ^ a[i])
      for i in range(2, (n + 2)):
          x2 = (x2 ^ i)
      return (x1 ^ x2)
source_cpp: int find_the_missing_number_2(vector<int> a, int n) {
    int x1 = a [ 0 ];
    int x2 = 1;
    for ( int i = 1;
    i < n;
    i ++ ) x1 = x1 ^ a [ i ];
    for ( int i = 2;
    i <= n + 1;
    i ++ ) x2 = x2 ^ i;
    return ( x1 ^ x2 );
  }