445. 两数相加 II - 码农教程

非进阶版本的三次翻转链表，这道题成功让我明白了我的脑子是有问题的，是没有逻辑可言的，做算法题是面向错误答案编程的。下次先要写伪代码才合理一点。

  1 ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
  2     if (l1 == nullptr)
  3         return l2;
  4     else if (l2 == nullptr)
  5         return l1;
  6     ListNode* dummy1 = nullptr;
  7     ListNode* dummy2 = nullptr;
  8     ListNode* temp1 = l1;
  9     ListNode* temp2 = l2;    
 10     ListNode* backup1 = new ListNode(0);
 11     while (temp1 != nullptr)
 12     {
 13         temp1 = temp1->next;
 14         l1->next = dummy1;
 15         dummy1 = l1;
 16         l1 = temp1;
 17     }
 18     l1 = dummy1;
 19     while (temp2 != nullptr)
 20     {
 21         temp2 = temp2->next;
 22         l2->next = dummy2;
 23         dummy2 = l2;
 24         l2 = temp2;
 25     }
 26     l2 = dummy2;
 27 
 28     bool carry = false;
 29     temp1 = l1;
 30     temp2 = l2;
 31     ListNode* pre_l1 = nullptr;
 32     ListNode* pre_l2 = nullptr;
 33     while (temp1!=nullptr && temp2!=nullptr)    //把新结果放在 l1中
 34     {
 35         if (carry == false && temp1->val + temp2->val < 10)    //之前无进位，相加小于10
 36         {
 37             temp1->val = temp1->val + temp2->val;
 38         }
 39         else if (carry == false && temp1->val + temp2->val >= 10)    //之前无进位，相加大于等于10
 40         {
 41             carry = true;
 42             temp1->val = temp1->val + temp2->val - 10;
 43         }
 44         else if(carry == true && temp1->val + temp2->val >= 9 )    //之前有进位，相加大于等于9
 45         {
 46             carry = true;
 47             temp1->val = temp1->val + temp2->val +1  - 10;
 48         }
 49         else    //之前有进位，相加小于9
 50         {
 51             carry = false;
 52             temp1->val = temp1->val + temp2->val+1;
 53         }
 54         pre_l1 = temp1;
 55         temp1 = temp1->next;
 56         pre_l2 = temp2;
 57         temp2 = temp2->next;
 58     }
 59 
 60     if (temp1 == nullptr&&temp2 == nullptr&&carry==true)
 61     {
 62         carry = false;
 63         backup1->val = 1;
 64         backup1->next = nullptr;
 65         pre_l1->next = backup1;        
 66     }
 67     else if (temp1 == nullptr&&carry == true)
 68     {
 69         pre_l1->next = temp2;
 70         while (temp2)
 71         {
 72             if (carry == true)
 73             {
 74                 temp2->val += 1;
 75                 carry = false;
 76             }
 77             if (temp2->val >= 10)
 78             {
 79                 carry = true;
 80                 temp2->val -= 10;
 81             }
 82             pre_l2 = temp2;
 83             temp2 = temp2->next;
 84         }
 85         if (carry == true)
 86         {
 87             backup1->val = 1;
 88             backup1->next = nullptr;
 89             pre_l2->next = backup1;
 90         }
 91     }
 92     else if (temp2==nullptr&&carry==true)    
 93     {
 94         while (temp1)
 95         {
 96             if (carry == true)
 97             {
 98                 temp1->val += 1;
 99                 carry = false;
100             }
101 
102             if (temp1->val >= 10)
103             {
104                 carry = true;
105                 temp1->val -= 10;
106             }
107             pre_l1 = temp1;
108             temp1 = temp1->next;
109         }
110         if (carry == true)
111         {
112             backup1->val = 1;
113             backup1->next = nullptr;
114             pre_l1->next = backup1;
115         }
116     }
117     else if (temp2 != nullptr&&carry==false)
118     {
119         pre_l1->next = temp2;
120     }
121     ListNode* dummy3 = nullptr;
122     ListNode* temp3 = l1;
123     while (temp3 != nullptr)
124     {
125         temp3 = temp3->next;
126         l1->next = dummy3;
127         dummy3 = l1;
128         l1 = temp3;
129     }
130     l1 = dummy3;
131     
132     return l1;
133 }

执行用时 :32 ms, 在所有 C++ 提交中击败了90.63%的用户

内存消耗 :9.8 MB, 在所有 C++ 提交中击败了99.54%的用户

祭奠我逝去的两个小时。。。

至于题解中看到的用栈，感觉学不到很多，不如用递归，虽然好像没什么区别，但用栈有点无脑。。虽然递归也是用到的栈

递归解法

 1 int length(ListNode* l1)
 2 {
 3     if (l1 == nullptr)
 4         return 0;
 5     else
 6         return 1 + length(l1->next);
 7 }
 8 int addTwoNumbersHelper(ListNode* l1, ListNode* l2, ListNode* &res)
 9 {
10     if (l1 == nullptr || l2 == nullptr)
11     {
12         return 0;
13     }
14     int sum = l1->val + l2->val + addTwoNumbersHelper(l1->next, l2->next,res);
15     ListNode* newhead = new ListNode(sum % 10);
16     newhead->next = res;
17     res = newhead;
18     return sum / 10;
19 }
20 ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
21     if (l1 == nullptr)
22         return l2;
23     else if (l2 == nullptr)
24         return l1;
25     int m = length(l1);
26     int n = length(l2);
27 
28     ListNode* res = nullptr;
29     ListNode* dummy = new ListNode(0);
30     int carry = 0;
31     if (m > n)
32     {
33         int k = m - n;
34         dummy->next = l2;
35         while (k--)
36         {
37             ListNode* makeup = new ListNode(0);
38             makeup->next = dummy->next;
39             dummy->next = makeup;
40         }
41         carry = addTwoNumbersHelper(l1, dummy->next, res);
42     }
43     else if (m < n)
44     {
45         int k = n - m;
46         dummy->next = l1;
47         while (k--)
48         {
49             ListNode* makeup = new ListNode(0);
50             makeup->next = dummy->next;
51             dummy->next = makeup;
52         }
53         carry = addTwoNumbersHelper(dummy->next, l2, res);
54     }
55     else
56     {
57         carry = addTwoNumbersHelper(l1, l2, res);
58     }
59     if (carry > 0)
60     {
61         ListNode* makeup = new ListNode(carry);
62         makeup->next = res;
63         res = makeup;
64     }
65     return res;
66 }

原文地址：https://www.cnblogs.com/zouma/p/11516641.html