LeetCode-in-All
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
import com_github_leetcode.ListNode
/*
* Definition for singly-linked list.
* class ListNode(_x: Int = 0, _next: ListNode = null) {
* var next: ListNode = _next
* var x: Int = _x
* }
*/
object Solution {
def swapPairs(head: ListNode): ListNode = {
if (head == null) {
return null
}
val len = getLength(head)
reverse(head, len)
}
private def getLength(curr: ListNode): Int = {
var cnt = 0
var current = curr
while (current != null) {
cnt += 1
current = current.next
}
cnt
}
private def reverse(head: ListNode, len: Int): ListNode = {
if (len < 2) {
return head
}
var curr = head
var prev: ListNode = null
var next: ListNode = null
for (i <- 0 until 2) {
next = curr.next
curr.next = prev
prev = curr
curr = next
}
head.next = reverse(curr, len - 2)
prev
}
}