PermCheck

A non-empty array A consisting of N integers is given.

A permutation is a sequence containing each element from 1 to N once, and only once.

For example, array A such that:
    A[0] = 4
    A[1] = 1
    A[2] = 3
    A[3] = 2

is a permutation, but array A such that:
    A[0] = 4
    A[1] = 1
    A[2] = 3

is not a permutation, because value 2 is missing.

The goal is to check whether array A is a permutation.

Write a function:

    int solution(vector<int> &A);

that, given an array A, returns 1 if array A is a permutation and 0 if it is not.

For example, given array A such that:
    A[0] = 4
    A[1] = 1
    A[2] = 3
    A[3] = 2

the function should return 1.

Given array A such that:
    A[0] = 4
    A[1] = 1
    A[2] = 3

the function should return 0.

Write an efficient algorithm for the following assumptions:

        N is an integer within the range [1..100,000];
        each element of array A is an integer within the range [1..1,000,000,000].

#include <set>

int solution(vector<int> &A) {
    // write your code in C++11 (g++ 4.8.2)
    set<int> s;
    int maximum = 1;
    for (auto i: A) {
        if (s.find(i) == s.end()) {
            s.insert(i);
        } else {
            return false;
        }
        maximum = max(i, maximum);
    }
    
    return (maximum == s.size());
}