Calculating Execution time and big O time of my c++ program - c++

Question

My Code is:
#include <iostream>
#include <utility>
#include <algorithm>
//#include <iomanip>
#include <cstdio>
//using namespace std;
inline int overlap(std::pair<int,int> classes[],int size)
{
std::sort(classes,classes+size);
int count=0,count1=0,count2=0;
int tempi,tempk=1;
for(unsigned int i=0;i<(size-1);++i)
{
tempi = classes[i].second;
for(register unsigned int j=i+1;j<size;++j)
{
if(!(classes[i].first<classes[j].second && classes[i].second>classes[j].first))
{ if(count1 ==1)
{
count2++;
}
if(classes[i].second == tempi)
{
tempk =j;
count1 = 1;
}
////cout<<"\n"<<"Non-Overlapping Class:\t";
////cout<<classes[i].first<<"\t"<<classes[i].second<<"\t"<<classes[j].first<<"\t"<<classes[j].second<<"\n";
classes[i].second = classes[j].second;
count++;
if(count1==1 && j ==(size-1))
{
j= tempk;
classes[i].second = tempi;
count1= 0;
if(count2 !=0)
{
count = (count + ((count2)-1));
}
count2 =0;
}
}
else
{
if(j ==(size-1))
{
if(count>0)
{
j= tempk;
classes[i].second = tempi;
count1= 0;
if(count2 !=0)
{
count = (count + ((count2)-1));
}
count2 =0;
}
}
}
}
}
count = count + size;
return count;
}
inline int fastRead_int(int &x) {
register int c = getchar_unlocked();
x = 0;
int neg = 0;
for(; ((c<48 || c>57) && c != '-'); c = getchar_unlocked());
if(c=='-') {
neg = 1;
c = getchar_unlocked();
}
for(; c>47 && c<58 ; c = getchar_unlocked()) {
x = (x<<1) + (x<<3) + c - 48;
}
if(neg)
x = -x;
return x;
}
int main()
{
int N;
////cout<<"Please Enter Number Of Classes:";
clock_t begin,end;
float time_interval;
begin = clock();
while(fastRead_int(N))
{
switch(N)
{
case -1 : end = clock();
time_interval = float(end - begin)/CLOCKS_PER_SEC;
printf("Execution Time = %f",time_interval);
return 0;
default :
unsigned int subsets;
unsigned int No = N;
std::pair<int,int> classes[N];
while(No--)
{
////cout<<"Please Enter Class"<<(i+1)<<"Start Time and End Time:";
int S, E;
fastRead_int(S);
fastRead_int(E);
classes[N-(No+1)] = std::make_pair(S,E);
}
subsets = overlap(classes,N);
////cout<<"\n"<<"Total Number Of Non-Overlapping Classes is:";
printf("%08d",subsets);
printf("\n");
break;
}
}
}
and Input and output of my program:
Input:
5
1 3
3 5
5 7
2 4
4 6
3
500000000 1000000000
1 5
1 5
1
999999999 1000000000
-1
Output:
Success time: 0 memory: 3148 signal:0
00000012
00000005
00000001
Execution Time = 0.000036
I tried to calculate by having clocks at start of main and end of main and found out the time.But it said only some 0.000036 secs.But when I tried to post the same code in Online Judge(SPOJ).My program got 'Time Limit Exceeded' Error. Time Limit for the above program in SPOJ is 2.365 secs.Could somebody help me figure out this?

Answer 1

I consider that your question is about the overlap function.
In it, you have
A sort call: O(n×ln(n))
two for loops:
the first is roughly 0..Size
the second (nested in the first) is roughly i..Size
The inside of the second loop is called Size(Size+1) / 2 (reverse sum of the N first integer) times with no breaks.
So your algorithm is O(n²) where n is the size.