24. Swap Nodes in Pairs

Problem

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list's nodes, only nodes itself may be changed.

Solution

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function(head) {
  var out = new ListNode(0);
  var now = out;

  out.next = head;

  while (now.next && now.next.next) {
    now = swap(now, now.next, now.next.next);
  }

  return out.next;
};

var swap = function (a, b, c) {
  a.next = c;
  b.next = c.next;
  c.next = b;
  return b;
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).