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This question was recently asked by Google.

Given two sorted linked lists, merge them in order.

Given two sorted linked lists, merge them in order.

# Definition for a linked-list node. class Node(object): def __init__(self, val, next=None): self.val = val self.next = next class Solution: def mergeTwoLists(self, a, b): # # Fill this in. # # Test program # 1 -> 3 ->5 a = Node(1) a.next = Node(3) a.next.next = Node(5) # 2 -> 4 -> 6 b = Node(2) b.next = Node(4) b.next.next = Node(6) c = Solution().mergeTwoLists(a, b) while c: print(c.val) c = c.next # 1 2 3 4 5 6 |

This problem can be solved recursively or iteratively. We traverse the two linked lists in parallel, advancing both pointers simultaneously. If the first list's value is smaller we advance that one, otherwise we advance the second list. If either list is shorter, then we take values from the longer list.

The time complexity is linear **O(n)** since both lists are traversed just once. The space complexity of the recursive algorithm is linear **O(n)**, since it builds up a recursive stack that may be as deep as the length of both lists. The space complexity of the iterative solution is constant **O(1)**, since only a few variables are used.

# Definition for a linked-list node. class Node(object): def __init__(self, val, next=None): self.val = val self.next = next class Solution: def mergeTwoLists(self, a, b): if a is None: return b elif b is None: return a elif a.val < b.val: a.next = self.mergeTwoLists(a.next, b) return a else: b.next = self.mergeTwoLists(a, b.next) return b def mergeTwoListsIterative(self, a, b): node = None head = None while True: if a is None: nextNode = b elif b is None: nextNode = a elif a.val < b.val: nextNode = a else: nextNode = b if nextNode == a: a = a.next if a else None if nextNode == b: b = b.next if b else None if nextNode is None: break if not node: node = nextNode head = node else: node.next = nextNode node = nextNode return head # Test program # 1 -> 3 ->5 a = Node(1) a.next = Node(3) a.next.next = Node(5) # 2 -> 4 -> 6 b = Node(2) b.next = Node(4) b.next.next = Node(6) c = Solution().mergeTwoLists(a, b) while c: print(c.val) c = c.next # 1 2 3 4 5 6

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TechLead

"TechLead" is Patrick Shyu - ex-Google/ex-Facebook Tech Lead, multi-millionaire app entrepreneur, digital nomad, Silicon Valley native, and senior software engineer. He's held roles in full-stack web development and mobile engineering. He has conducted over 100 interviews at Google, and has worked in the tech industry for over a decade from startups to Fortune 500 companies.