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24. Swap Nodes in Pairs
Medium
Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list’s nodes (i.e., only nodes themselves may be changed.)
Example 1:
Input: head = [1,2,3,4]
Output: [2,1,4,3]
Example 2:
Input: head = []
Output: []
Example 3:
Input: head = [1]
Output: [1]
Constraints:
- The number of nodes in the list is in the range
[0, 100]. 0 <= Node.val <= 100
Solution
/**
* 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 swapPairs(head: ListNode | null): ListNode | null {
if (head === null) {
return null
}
const len = getLength(head)
return reverse(head, len)
}
function getLength(curr: ListNode | null): number {
let cnt = 0
while (curr !== null) {
cnt++
curr = curr.next
}
return cnt
}
// Recursive function to reverse in groups
function reverse(head: ListNode | null, len: number): ListNode | null {
// base case
if (len < 2) {
return head
}
let curr: ListNode | null = head
let prev: ListNode | null = null
let next: ListNode | null
for (let i = 0; i < 2; i++) {
// reverse linked list code
next = curr.next
curr.next = prev
prev = curr
curr = next
}
head.next = reverse(curr, len - 2)
return prev
}
export { swapPairs }