445. Add Two Numbers II

stack的方式，把两个输入都用栈存起来，计算出的数值也用栈存起来

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1:
            return l2
        if not l2:
            return l1
        carry = 0
        stack1 = []
        stack2 = []
        q1 = l1
        q2 = l2
        while q1:
            stack1.append(q1)
            q1 = q1.next
        while q2:
            stack2.append(q2)
            q2 = q2.next
        stack3 = []
        while stack1 and stack2:
            sum = stack1.pop().val + stack2.pop().val + carry
            he = sum % 10
            carry = sum / 10
            stack3.append(he)
        while stack1:
            sum = stack1.pop().val + carry
            he = sum % 10
            carry = sum / 10
            stack3.append(he)
        while stack2:
            sum = stack2.pop().val + carry
            he = sum % 10
            carry = sum / 10
            stack3.append(he)
        if carry:
            stack3.append(carry)
        dummy = ListNode(0)
        p = dummy
        while stack3:
            p.next = ListNode(stack3.pop())
            p = p.next
        return dummy.next
    

另外一种有趣的做法

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):

        x1, x2 = 0, 0
        while l1:
            x1 = x1*10+l1.val
            l1 = l1.next
        while l2:
            x2 = x2*10+l2.val
            l2 = l2.next
        x = x1 + x2
        
        head = ListNode(0)
        if x == 0: return head
        while x:
            v, x = x%10, x//10
            head.next, head.next.next = ListNode(v), head.next
            
        return head.next
            

网上的一种做法，很投机但很有趣

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):

        x1, x2 = 0, 0
        while l1:
            x1 = x1*10+l1.val
            l1 = l1.next
        while l2:
            x2 = x2*10+l2.val
            l2 = l2.next
        x = x1 + x2
        
        head = ListNode(0)
        if x == 0: return head
        while x:
            v, x = x%10, x//10
            head.next, head.next.next = ListNode(v), head.next
        return head.next