解析

//! n,1 -> 遍历两个链表耗时n1+n2，不记返回值占地常数级
class Solution {
public:
  ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
    ListNode *node1 = l1;
    ListNode *node2 = l2;
    int base = 0, carry = 0, num1 = 0, num2 = 0;
    ListNode *node = new ListNode(-1);
    ListNode *answer = node;
    while (node1 != nullptr || node2 != nullptr) {
      num1 = 0, num2 = 0;
      if (node1 != nullptr) {
        num1 = node1->val;
        node1 = node1->next;
      }
      if (node2 != nullptr) {
        num2 = node2->val;
        node2 = node2->next;
      }
      base = (num1 + num2 + carry) % 10;
      node->next = new ListNode(base);
      node = node->next;
      carry = (num1 + num2 + carry) / 10;
    }
    if(carry!=0){
        node->next =new ListNode(carry);
    }
    return answer->next;
  }
};