problem

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

 

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

 

Constraints:

  • The number of nodes in both lists is in the range [0, 50].
  • -100 <= Node.val <= 100
  • Both list1 and list2 are sorted in non-decreasing order.

submission

// Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option<Box<ListNode>>
// }
// 
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//     ListNode {
//       next: None,
//       val
//     }
//   }
// }

// easier to solve with recursion
// pick the smaller node and merge the rest recursively
impl Solution {
    pub fn merge_two_lists(
        list1: Option<Box<ListNode>>,
        list2: Option<Box<ListNode>>,
    ) -> Option<Box<ListNode>> {
        // unfortunately we can't simplify this with let-else
        match (list1, list2) {
            (Some(list1), Some(list2)) => {
                // choose a smaller node as head
                // recursively merge the rest
                let (mut min, max) = match list1.val < list2.val {
                    true => (list1, list2),
                    false => (list2, list1),
                };
                min.next = Self::merge_two_lists(min.next, Some(max));
                Some(min)
            }
            // both Option::xor & Option::or works here
            (list1, list2) => list1.xor(list2),
        }
    }
}