singularity

singularity 🔎

21. ✅ Merge Two Sorted Linked Lists

O(n)

naive, iterative, looping, uses dummy nodes

var mergeTwoLists = function(list1, list2) {

    let newHead = dummyHead = new ListNode(); 

    while(list1 && list2){
        if (list1.val < list2.val){
            dummyHead.next = list1;
            list1 = list1.next;
        } else {
            dummyHead.next = list2;
            list2 = list2.next;
        }
        dummyHead = dummyHead.next;
    }  

    if(list1){
        dummyHead.next = list1;
    }
    if(list2){
        dummyHead.next = list2;
    }    

    return newHead.next;
};

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        newHead = dummyHead = ListNode()
        
        while list1 and list2:
            if list1.val < list2.val:
                dummyHead.next = list1
                list1 = list1.next
            else:
                dummyHead.next = list2
                list2 = list2.next
            dummyHead = dummyHead.next
        
        dummyHead.next = list1 if list1 else list2 

        return newHead.next

recursive

• unoptimised tail-recursion in python (1000 calls), but we can try

class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:

        if not list1:
            return list2

        if not list2:
            return list1 

        if list1.val <= list2.val:
            list1.next = self.mergeTwoLists(list1.next, list2)
            return list1
        else:
            list2.next = self.mergeTwoLists(list1, list2.next)
            return list2

impl Solution {
    pub fn merge_two_lists(list1: Option<Box<ListNode>>, list2: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
        match (list1, list2) {
            (None, None) => None,
            (Some(l1), None) => Some(l1),
            (None, Some(l2)) => Some(l2),
            (Some(mut l1), Some(mut l2)) => {
                if l1.val <= l2.val {
                    l1.next = Self::merge_two_lists(l1.next, Some(l2));
                    Some(l1)
                } else {
                    l2.next = Self::merge_two_lists(Some(l1), l2.next);
                    Some(l2)
                }
            }
        }
    }
}

• [two pointers]