this algo with O(n*log(n))
but i don't know why WA on Test21, please help me!

[code deleted]

Edited by moderator 24.11.2019 13:39

Your mistakes:
a[100001] must be a[500001]
a[(n-1)/2] must be a[(n+1)/2]
qsort(a,n,sizeof(int),compare) must be qsort(a,n+1,sizeof(int),compare).

So AC is here:
[code deleted]

Edited by moderator 24.11.2019 13:38