LeetCode—兩數相加

題目來自LeetCode：

https://leetcode-cn.com/problems/add-two-numbers/description/

注意幾點：

1. 鏈表對應結點相加時增加前一個結點的進位，並保存下一個結點的進位；
2. 兩個鏈表長度不一致時，要處理較長鏈表剩餘的高位和進位計算的值；
3. 如果最高位計算時還產生進位，則還需要添加一個額外結點。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode * p1,* p2,* p3,* l3, *temp;   
    p1 = l1;
    p2 = l2;
    l3 = (struct ListNode *)malloc(sizeof(struct ListNode));
    p3 = l3;
    int carry = 0; //進位
    int sum; 
    while (p1 && p2) {
        temp = (struct ListNode *)malloc(sizeof(struct ListNode));    //創建新節點
        p3->next = temp;
        temp->next = NULL;
        sum = p1->val + p2->val + carry;
        if(sum >= 10) {
            carry = 1;
            temp->val = sum%10;
        }else {
            carry = 0;
            temp->val = sum;
        }
        p1=p1->next;
        p2=p2->next;
        p3=p3->next;
    }
    
    while (p1) {
        temp = (struct ListNode *)malloc(sizeof(struct ListNode));    //創建新節點
        p3->next = temp;
        temp->next = NULL;
        sum = p1->val +  carry;
        if(sum >= 10) {
            carry = 1;
            temp->val = sum%10;
        }else {
            carry = 0;
            temp->val = sum;
        }
        p1=p1->next;
        p3=p3->next;
    }
    
    while ( p2) {
        temp = (struct ListNode *)malloc(sizeof(struct ListNode));    //創建新節點
        p3->next = temp;
        temp->next = NULL;
        sum = p2->val + carry;
        if(sum >= 10) {
            carry = 1;
            temp->val = sum%10;
        }else {
            carry = 0;
            temp->val = sum;
        }
        p2=p2->next;
        p3=p3->next;
    }
    
    if(carry == 1) {
        temp = (struct ListNode *)malloc(sizeof(struct ListNode));    //創建新節點
        p3->next = temp;
        temp->next = NULL;
        temp->val = 1;
    }
    
    
    return l3->next;
}