When I was solving the problem [674G — Choosing Ads](https://mirror.codeforces.com/problemset/problem/674/G), I got a TLE on #10 as result. The submission id is [348821830](https://mirror.codeforces.com/contest/674/submission/348821830).↵
↵
I tried to generate a data and run my code on my Ubuntu, and it took 5.80 seconds. (The python code to generate the data is at the end of this blog.) ↵
↵
I was once told that O2 can make the code run faster, so I tried to compile the code with option `-O2` (the full command was like `g++ ./t.cpp -o ./t -O2`), it just took 0.73s. I added `#pragma GCC optimize(2)` into the code, and then recompiled it without option `-O2` (the full command was like `g++ ./t.cpp -o ./t`), however it took more than 5 seconds. (I used the command `/usr/bin/time -v ./t < ./t.in > ./t.out` to measure the time.)↵
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As you can see the two ways above has a huge difference.↵
↵
I'm wondering why this phenomenon happened.↵
↵
Is there some problem in my code or the way I compile it? or it can just happen on my Ubuntu?↵
↵
Thanks a lot.↵
↵
---↵
↵
```python↵
#!/bin/python3↵
↵
from random import *↵
↵
n=150000↵
m=150000↵
k=20↵
↵
print(n,m,k)↵
for _ in range(n):↵
print(randint(1,6),end=' ')↵
print()↵
for _ in range(m):↵
o=randint(1,2)↵
l=randint(1,n)↵
r=randint(1,n)↵
v=randint(1,6)↵
if l>r:↵
l,r=r,l↵
if o==1:↵
print(o,l,r,v)↵
else:↵
print(o,l,r)↵
↵
```↵
↵
When I was testing on my Ubuntu, all tests generated by this code will cause the phenomenon above.↵
↵
---↵
↵
Update↵
------↵
↵
I've put `#pragma GCC optimize(2)` to the first line, then the time reduced to 3.13 seconds, still far more than `-O2`.↵
↵
---↵
↵
Update #2↵
------↵
↵
I've passed it. I changed the map and vector to simple arrays like `a[150005]`, which seems to be a good way to improve speed. On the local test, it took just about 0.5 second. But why there was such a huge difference on `pragma` and `-O2`?
↵
I tried to generate a data and run my code on my Ubuntu, and it took 5.80 seconds. (The python code to generate the data is at the end of this blog.) ↵
↵
I was once told that O2 can make the code run faster, so I tried to compile the code with option `-O2` (the full command was like `g++ ./t.cpp -o ./t -O2`), it just took 0.73s. I added `#pragma GCC optimize(2)` into the code, and then recompiled it without option `-O2` (the full command was like `g++ ./t.cpp -o ./t`), however it took more than 5 seconds. (I used the command `/usr/bin/time -v ./t < ./t.in > ./t.out` to measure the time.)↵
↵
As you can see the two ways above has a huge difference.↵
↵
I'm wondering why this phenomenon happened.↵
↵
Is there some problem in my code or the way I compile it? or it can just happen on my Ubuntu?↵
↵
Thanks a lot.↵
↵
---↵
↵
```python↵
#!/bin/python3↵
↵
from random import *↵
↵
n=150000↵
m=150000↵
k=20↵
↵
print(n,m,k)↵
for _ in range(n):↵
print(randint(1,6),end=' ')↵
print()↵
for _ in range(m):↵
o=randint(1,2)↵
l=randint(1,n)↵
r=randint(1,n)↵
v=randint(1,6)↵
if l>r:↵
l,r=r,l↵
if o==1:↵
print(o,l,r,v)↵
else:↵
print(o,l,r)↵
↵
```↵
↵
When I was testing on my Ubuntu, all tests generated by this code will cause the phenomenon above.↵
↵
---↵
↵
Update↵
------↵
↵
I've put `#pragma GCC optimize(2)` to the first line, then the time reduced to 3.13 seconds, still far more than `-O2`.↵
↵
---↵
↵
Update #2↵
------↵
↵
I've passed it. I changed the map and vector to simple arrays like `a[150005]`, which seems to be a good way to improve speed. On the local test, it took just about 0.5 second. But why there was such a huge difference on `pragma` and `-O2`?
