24. Swap Nodes in Pairs

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

 

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

 

Follow up: Can you solve the problem without modifying the values in the list's nodes? (i.e., Only nodes themselves may be changed.)

Rust Solution

struct Solution;
use rustgym_util::*;

impl Solution {
    fn swap_pairs(head: ListLink) -> ListLink {
        if let Some(mut first) = head {
            if let Some(mut second) = first.next.take() {
                let rest = second.next.take();
                first.next = Self::swap_pairs(rest);
                second.next = Some(first);
                Some(second)
            } else {
                Some(first)
            }
        } else {
            None
        }
    }
}

#[test]
fn test() {
    let head = list!(1, 2, 3, 4);
    let res = list!(2, 1, 4, 3);
    assert_eq!(Solution::swap_pairs(head), res);
}