两数相加 注意点：在链表没有遍历完或者还有余数没有加上的时候，都要继续进行。
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode() : val(0), next(nullptr) {} * ListNode(int x) : val(x), next(nullptr) {} * ListNode(int x, ListNode *next) : val(x), next(next) {} * }; */ class Solution { public: ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) { ListNode* pre = new ListNode,*cur = pre; // pre 是哨兵 int t = 0; while(l1 && l2){ t += l1->val + l2->val; l1 = l1->next; l2 = l2->next; ListNode* node = new ListNode(t%10); cur->next = node; cur = node; t /= 10; } ListNode* l = l1?l1:l2; while(l){ t += l->val; l = l->next; ListNode* node = new ListNode(t%10); cur->next = node; cur = node; t/= 10; } while(t){ ListNode* node = new ListNode(t%10); cur->next = node; cur = node; t/= 10; } return pre->next; } };