Find the missing element in a given permutation.

## Question

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(NSMutableArray *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.

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## Solution

- (int)permMissing:(NSMutableArray*) A { //(2, 3, 1, 5) //(2, 3, 4, 5, 6) //(3, 2) //(1) //(4, 2, 3, 5, 1, 6, 8, 9) int size = A.count; //4 int sum = 0; int add = 0; int missing_num = 0; int i; for(i = 0; i < size + 1; i ++){ if(i < size) sum = sum + [A[i] integerValue]; add = add + i+1; } missing_num = add - sum; return missing_num; }

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