make it clear this way:

1) temp1 & temp2: initialized
2) Then pair comparation。For instance, a1&an, a2&an-2…ai&an-1 n/2 times
3) when each pair comparation ，if the larger >temp1，->temp1，n/2 times； if the smaller ＜temp2，->temp2，n/2 times；
so total=3/2n