Lesson 3 Time Complexity PermMissingElem

    Codility 测试题 寻找缺失的元素。

  原题：

           

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(int A[], int N);

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.

 

    算法 

       

     func findMissElem( needArray A: [NSInteger] )->NSInteger{  

        var sumACount = (A.count + 1)*(A.count + 2)/2

        for scorein A{

            sumACount -= score

        }

        return sumACount

     }