What is selection sort? It is an algorithm to order elements. You have an original unordered array, and to order it you build a new one. You put elements on the new one iterating on the original, finding at each iteration the smallest number there, and putting the smallest element on the new array.
What is the Big-O notation for selection sort? Since you have an unordered list with n elements, and you pass at each one of them n number of times, the Big O notation for this algorithm is: O₍n x n), which is equal to O₍n²₎ .
Why is selection sort Big-O notation O₍n²₎, if at each pass through all the original array you remove the smallest element of it, and at the last step there is only one element? That is because Big-O notation only works with integer numbers. So, since there is no way to represent n * 1/2n , you have to round 1/2n to n, getting n * n, which equals n².
Is Selection Sort the fastest sorting algorithm? If not, is there an example of a faster one? No, it is not. Quicksort is an example of a faster one.