两个链表相加

1 题目
You are giventwo linked lists representing two non-negative numbers. The digits are storedin reverse order
and each of their nodes contain a single digit. Add the twonumbers and return it as a linked list.
Input: (2 -> 4 -> 3)+ (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
2 分析
该题目属于链表的相加，需要注意的有：
(1) 考虑两个链表的长度，尤其是链表为空时也能处理。
(2) 每个结点只表示一位数字。
(3) 当链表末尾结点相加后若有进位，则需要申请新的结点存储信息。

3 核心代码：

int listLength(ListNode* head)  //递归得到链表的长度
{
    return head ? 1 + listLength(head ->next) : 0;
}  

ListNode *addTwoNumbers(ListNode *l1, ListNode *l2){  //把两个链表相加(前面的参数为长链表)
    if (listLength(l1) < listLength(l2)){
        return addTwoNumbers(l2, l1);
    }
    ListNode *head1 = l1, *head2 = l2;
    int inc = 0;
    bool isEnd = false;
    while (head2){
        int val = head1 -> val + head2-> val + inc;
        head1 -> val = val % 10;
        inc = val / 10;
        if (head1 -> next){
            head1 = head1 -> next;
        }else{
            isEnd = true;
        }
        head2 = head2 -> next;
    }
    while (inc){ //当短链表计算完之后
        int val = isEnd ? inc : head1 ->val + inc;
        if (isEnd){
            head1 -> next = new ListNode(val % 10);
        }else{
            head1 -> val = val % 10;
        }
        inc = val / 10;
        if (head1 -> next){
            head1 = head1 -> next;
        }else{
            isEnd = true;
        }
    }
    return l1;
}

4 以下为代码的完整实现

#include "stdafx.h"
#include<string.h>
#include<iostream>

using namespace std;
struct ListNode {  //sizeof(ListNode):8
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
}; 

int listLength(ListNode* head)  //递归得到链表的长度
{
    return head ? 1 + listLength(head ->next) : 0;
}  

ListNode *addTwoNumbers(ListNode *l1, ListNode *l2){  //把两个链表相加(前面的参数为长链表)
    if (listLength(l1) < listLength(l2)){
        return addTwoNumbers(l2, l1);
    }
    ListNode *head1 = l1, *head2 = l2;
    int inc = 0;
    bool isEnd = false;
    while (head2){
        int val = head1 -> val + head2-> val + inc;
        head1 -> val = val % 10;
        inc = val / 10;
        if (head1 -> next){
            head1 = head1 -> next;
        }else{
            isEnd = true;
        }
        head2 = head2 -> next;
    }
    while (inc){ //当短链表计算完之后
        int val = isEnd ? inc : head1 ->val + inc;
        if (isEnd){
            head1 -> next = new ListNode(val % 10);
        }else{
            head1 -> val = val % 10;
        }
        inc = val / 10;
        if (head1 -> next){
            head1 = head1 -> next;
        }else{
            isEnd = true;
        }
    }
    return l1;
}
//******************************************************************
ListNode *createList(int arr[],int len){  //根据数组创建链表
    ListNode* head = NULL,*p;
    p=head;
    for(int i=0; i<len; i++){
        if(i==0){
            head = new ListNode(arr[i]);
            p = head;
        }else{
            p->next = new ListNode(arr[i]);
            p = p->next;
        }
    }
    return head;
}
void printList(ListNode *head){ //输出链表
    ListNode *p = head;
    if(head==NULL) cout<<"空链表"<<endl;
    else{
        while(p){
            cout<<p->val<<" ";
            p = p->next;
        }
        cout<<endl;
    }
}
int main(int argc, char * argv[])
{   //32位系统中所有指针(包括复杂结构体)均为4个字节，float、long和int类型为4个字节，char为1个字节，double为8个字节
    //char arr[] = "12345678";        //strlen:8  sizeof:9  sizeof(arr)/sizeof(arr[0]):9
                      //作为参数时      strlen:8  sizeof:4  sizeof(arr)/sizeof(arr[0]):4
    //int arr[] = {1,2,3,4,5,6,7,8};  //strlen:x  sizeof:32  sizeof(arr)/sizeof(arr[0]):8
                      //作为参数时      strlen:x  sizeof:4  sizeof(arr)/sizeof(arr[0]):1

    int arr1[] = {2, 4, 3};  //表示从低位到高位
    int arr2[] = {5, 6, 4};
    int len1 = sizeof(arr1)/sizeof(arr1[0]);
    int len2 = sizeof(arr2)/sizeof(arr2[0]);
    ListNode *l1 = createList(arr1,len1);
    ListNode *l2 = createList(arr2,len2);

    printList(addTwoNumbers(l1,l2));

    system("pause");
    return 0;
}