Let's take the most basic linked list first-- one with two nodes. We need to reverse this.

Seems pretty easy, right? To reverse an entire linked list, simply reverse every pointer. If 1 is pointing at 2, flip it so 2 should point to 1.

Of course, there's complexities that lie ahead. But for now, that's the basis for:

117 -> 2 -> 21 -> 6 -> 42 -> 10

becoming

117 <- 2 <- 21 <- 6 <- 42 <- 10