You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

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

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

Solution: Simulation

Using a stack to “reverse” the list. Simulate the addition digit by digit.

Time complexity: O(l1 + l2)

Space complexity: O(l1 + l2)

C++

// Author: Huahua
// Running time: 40 ms
class Solution {
public:
  ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
    stack<int> s1;
    stack<int> s2;
    while (l1) {
      s1.push(l1->val);
      l1 = l1->next;
    }    
    while (l2) {
      s2.push(l2->val);
      l2 = l2->next;
    }    
    ListNode* head = nullptr;
    int sum = 0;
    while (!s1.empty() || !s2.empty() || sum) {
      sum += s1.empty() ? 0 : s1.top();
      sum += s2.empty() ? 0 : s2.top();
      if (!s1.empty()) s1.pop();
      if (!s2.empty()) s2.pop();            
      ListNode* n = new ListNode(sum % 10);
      sum /= 10;
      n->next = head;
      head = n;      
    }    
    return head;      
  }
};

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