pls tell me where i am mistaking in my logic:
if u take l=k-1 then
whatever the first player takes the second player wins but on test 3 it says wrong anwser

Admin's tests aren`t wrong! (it is strange why they didn't ban you:). If you read task attentive, you may saw next thing: if there are few correct L, you must write smallest of them. And as you understand, L=k-1 is not smallest in all tests.