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## Discussion of Problem 1173. Lazy Snail

Why I always WA on text 2?
Posted by beststu 8 Jul 2008 13:54
Why I always WA on text 2?
Re: Why I always WA on text 2?
Posted by Fyodor Menshikov 24 Apr 2009 13:10
If you sort points by angle from Wally's home, there is one tricky case - when angle difference between two sequential points is greater than PI.

For example if angle range is [-PI; PI]:
0 0
3
-1 -1 1
0 1 2
-1 1 3
Program that sort angles will output
0
1
2
3
0
and it is wrong, 1-2 will intersect with 3-0.

Text for angle range [0; 2PI] is
0 0
3
1 1 1
0 -1 2
1 -1 3
Again sorting program will output
0
1
2
3
0
and again it is wrong, 1-2 will intersect with 3-0.

Hint: after sorting there may be not more than one pair of sequential points with angle difference greater than PI.
Re: Why I always WA on text 2?
Posted by wangbicheng1 18 Nov 2015 20:24
Thanks for the hint!