今天的周赛题，真的是听不懂，求教
DescriptionSorting is one of the most frequently performed computational tasks. Consider the special sorting problem in which the records to be sorted have at most three different key values. This happens for instance when we sort medalists of a competition according to medal value, that is, gold medalists come first, followed by silver, and bronze medalists come last.
In this task the possible key values are the integers 1, 2 and 3. The required sorting order is non-decreasing. However, sorting has to be accomplished by a sequence of exchange operations. An exchange operation, defined by two position numbers p and q, exchanges the elements in positions p and q.
You are given a sequence of key values. Write a program that computes the minimal number of exchange operations that are necessary to make the sequence sorted.
Input
Line 1: N (1 <= N <= 1000), the number of records to be sorted
Lines 2-N+1: A single integer from the set {1, 2, 3}
Output
A single line containing the number of exchanges required
Sample Input
9
2
2
1
3
3
3
2
3
1
Sample Output
4
Source
，看明白了，但是为什么总是要最后一个，拿最后一个来交换为什呢是最少次数。。。
。