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painless

#### PermCheck

Check whether array A is a permutation.
Programming language:

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 = 4 A = 1 A = 3 A = 2

is a permutation, but array A such that:

A = 4 A = 1 A = 3

is not a permutation, because value 2 is missing.

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

Write a function:

class Solution { public int solution(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 = 4 A = 1 A = 3 A = 2

the function should return 1.

Given array A such that:

A = 4 A = 1 A = 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].