[LeetCode] Visualization of Reverse Linked List

This is my memo when I solved 206. Reverse Linked List.

What we want

This is pretty simple. We want to re-order a provided linked list in the opposite direction.

Strategy

Go through each element and replace the value of next from the next element’s address with the previous element’s.

This requires us to handle two operations: 1) changing the value of next of each element and 2) tracking which element we’re updating and what element comes next.

Code

I think there are two tricky things when it comes to coding: 1) storing the next element to modify before updating curr.next, and 2) returning prev instead of curr as we update curr at the last of the while loop.

    /**
     * Definition for singly-linked list.
     * class ListNode {
     *     val: number
     *     next: ListNode | null
     *     constructor(val?: number, next?: ListNode | null) {
     *         this.val = (val===undefined ? 0 : val)
     *         this.next = (next===undefined ? null : next)
     *     }
     * }
     */

    function reverseList(head: ListNode | null): ListNode | null {
        let [prev, curr] = [null, head]
        while(curr){
          const next = curr.next
          curr.next = prev
          prev = curr
          curr = next
        }
        return prev

    };