206. Reverse Linked List

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time: O(n)

space: O(1)

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    let p = null;
    let c = head;

    while (c !== null) {
        const n = c.next;
        c.next = p;
        p = c;
        c = n;
    }

    return p;
};