[LeetCode]2. Add Two Numbers_Medium tag: Linked List

 

 

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

 

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

 

Constraints:

• The number of nodes in each linked list is in the range [1, 100].
• 0 <= Node.val <= 9
• It is guaranteed that the list represents a number that does not have leading zeros.

 

Ideas: use a pre (init: 0) to add into the curSum. Rember to check the pre after adding everything, because assume 9 + 1 -> 10. the 1 is saved in pre. 

T: O(m + n), S: O(1)   m: len(l1), n: len(l2)

 

Code:

dummy = ListNode()
        head = dummy
        pre = 0
        while l1 or l2:
            curSum = 0
            if l1:
                curSum += l1.val
                l1 = l1.next
            if l2:
                curSum += l2.val
                l2 = l2.next
            curSum += pre
            pre, rem = divmod(curSum, 10)
            newNode = ListNode(rem)
            head.next = newNode
            head = head.next
        if pre: head.next = ListNode(pre)
        return dummy.next