Java Bien!

For a project, i had to write a piece of code that would count for each day of a year how many classrooms of a university are used. We know the start day of use and the end days of use for each classroom. There can be millions of use for periods from 1 day to the whole year. The first alogrithm is came up with was this one :
[java]int[] used = new int [SIZE] ;

for (int i = 0; i < N; i++)
{
int start = starts [i] ;
int end = ends [i] ;

for (int j = start; j <= end; j++)
{
used [j]++ ;
}
}[/java]
Then I remembered some optimizing tips that my father's got from his job at Control Data years ago and I wrote this piece of code :
[java]int[] used = new int [SIZE + 1] ; // HACK
for (int i = 0; i < N; i++)
{
used [starts [i]]++ ;
used [ends [i] + 1]-- ;
}

int add = used [0] ;
for (int i = 1; i < SIZE; i++)
{
add += used [i] ;
used [i] = add ;
}[/java]

The second code sample is 35x faster than the first one when SIZE is 365 and N is 10000000. The difference is huge but the time taken by the first code is still good enough. Memory access was more expensive on a Control Data machine than nowadays...

CDC 7600 Franlab Paris (1973) 5 MBytes

CDC Cyber205 Minneapolis (1981) 32 MBytes