0023-Merge-k-Sorted-Lists.py

'''
Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Example:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6
'''

# Definition for singly-linked list.

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

# Method 1

class Solution:
    
    def mergeTwoLists(self, l1, l2):
        res = cur = ListNode()
        while l1 and l2:
            if l1.val <= l2.val:
                cur.next = l1
                l1 = l1.next
            else:
                cur.next = l2
                l2 = l2.next
            cur = cur.next
        cur.next = l1 or l2
        return res.next
        
    def mergeKLists(self, lists: List[ListNode]) -> ListNode:
        if not lists:
            return None
        last = len(lists) - 1
        while last:
            i, j = 0, last
            while i < j:
                lists[i] = self.mergeTwoLists(lists[i], lists[j])
                i += 1
                j -= 1
                last -= 1
        return lists[0]

# Method 2

class Solution:
    def mergeKLists(self, lists: List[ListNode]) -> ListNode:
        if not lists:
            return None
        last = len(lists) - 1
        while last:
            i, j = 0, last
            while i < j:
                res = cur = ListNode()
                first, second = lists[i], lists[j]
                while first and second:
                    if first.val <= second.val:
                        cur.next = first
                        first = first.next
                    else:
                        cur.next = second
                        second = second.next
                    cur = cur.next
                cur.next = first or second
                lists[i] = res.next
                i += 1
                j -= 1
                last -= 1
        return lists[0]