2. Add Two Numbers

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.

• we need to deal with one situation here:

l1     1     3                              1     
+      +     +                              +
l2     1     9                              1 
-----------------------------------------------------------------------------------
cur    2    12(we need to put 2 here)        2+carry(carry from the previous 12)
            12 %10 = 2
		    carry = 12//10 =1	   

By adding a variable called carry can deal with this situation: align the addends vertically and add the columns, starting from the left-most column. If a column’s sum exceeds nine, the extra digit “carried” add into the next column.

l1          1                  3                       1     
+           +                  +                       +
l2          1                  9                       1 
-----------------------------------------------------------------------------------
carry  0+1+1=3             0+3+9=12              1+1+1 = 3  
cur     3%10=3             12%10=2               3%10 = 3
carry  3//10 =0             12//10=1             3//10 = 0