swapNodesInPairs.js

class ListNode {
  constructor(val) {
    this.val = val;
    this.next = null;
  }
}

// const swapPairs = head => {
//   if (!head || !head.next) return head;
//   let prev = head;
//   let curr = head;
//   const newHead = curr.next;
//   if (!head.next.next || !head.next.next.next) {
//     const next = curr.next;
//     const temp = next.next;
//     next.next = curr;
//     curr.next = temp;
//     return newHead;
//   }
//   while (curr && curr.next) {
//     const next = curr.next;
//     const temp = next.next;
//     next.next = curr;
//     curr.next = temp;
//     prev.next = next;
//     prev = curr;
//     curr = temp;
//   }
//   return newHead;
// };

const swapPairs = head => {
  if (!head) return head;;

  let mid = head;
  let left = null;
  let right = head.next;
  let newHead = head;

  while (mid && right) {
    if (left) {
      left.next = right;
    } else {
      newHead = right;
    }
    mid.next = right.next;
    right.next = mid;

    left = mid;
    mid = !mid ? mid : mid.next;
    right = !mid ? mid : mid.next;
  }
  return newHead;
};

/*
specification
input: head of linked list
output: head of linked list after swapping each pair of nodes
constraints: constant extra space, don't modify value in the list's nodes
edge cases:

justification:
to swap pairs in a linked list

explanation
the input linked list will determine the order of the output linked list

visualization

approximation
break connections and reset them

verification

implementation

*/

const a1 = new ListNode(1);
const a2 = new ListNode(2);
const a3 = new ListNode(3);
const a4 = new ListNode(4);
a1.next = a2;
a2.next = a3;
a3.next = a4;

console.log(swapPairs(a1));