leetcode 160

i can't believe it's easy class. it's so hard for me, yesterday afternoon i feel there's no way to solve it except with complexity m*n. today i struggled all day but failed to find a way of complexity m+n. i feel so frustrated then i checked the solution after 15:00. jesus, i should check the solution earlier since i can never thought of switch head when hit the end. what's that? that's out of the common human ability like me.
as for the length difference way, that's also kind of weird for me, cause there's no clue in the problem description the two list intersected till the end, though in all the example they do intersected till the end.