leetcode 2. 两数相加

链表的应用

题目的思路很简单，通过建立一条新的链表用来存放两数相加的结果。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */


struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2){
    struct ListNode *p1 = l1, *p2 = l2;
    struct ListNode *ans = (struct ListNode *)malloc(sizeof(struct ListNode));
    struct ListNode *add = ans;
    int flag = 0;

    while(p1 || p2 || flag > 0){
        struct ListNode *num = (struct ListNode *)malloc(sizeof(struct ListNode));
        int x1 = p1 == NULL ? 0 : p1->val;
        int x2 = p2 == NULL ? 0 : p2->val;
        num->val = (x1 + x2 + flag) % 10;
        flag = (x1 + x2 + flag) / 10;
        num->next = NULL;

        ans->next = num;
        ans = num;

        if(p1)
            p1 = p1->next;
        if(p2)
            p2 = p2->next;
    }
    return add->next;
}