2. Add Two Numbers

题目

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order 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.

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

想法

定义一个进位变量，创建一个链表。 仿照数学中对两个整数的相加，将和放在新建的链表里。   

代码

struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {            struct ListNode * sum = malloc( sizeof( struct ListNode ) ), *coSum = sum;        int carry = 0;        while( 1 )    {            coSum->val = ( l1 ? 0 : l1->val ) + ( l2  ? 0 : l2->val ) + carry;       l1 = l1  ? NULL : l1->next;       l2 = l2 ? NULL : l2->next;                     carry = coSum->val / 10;       coSum->val %= 10;              if( l1  || l2  || carry != 0 )        {            coSum->next = malloc( sizeof( struct ListNode ) );            coSum = coSum->next;           }       else       {           coSum->next = NULL;              return sum;           }    }    }