題目：

給定兩個非空鏈表來表示兩個非負整數。位數按照逆序方式存儲，它們的每個節點只存儲單個數字。將兩數相加返回一個新的鏈表。

你可以假設除了數字 0 之外，這兩個數字都不會以零開頭。

示例：

輸入：(2 -> 4 -> 3) + (5 -> 6 -> 4)
輸出：7 -> 0 -> 8
原因：342 + 465 = 807

　解法：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if(l1 == NULL){
            return l2;
        }
        if(l2 == NULL){
            return l1;
        }
        
        ListNode * head = NULL;
        ListNode * pre = NULL;
        int carry = 0;
        while(l1&&l2){
            ListNode * newNode = new ListNode((l1->val + l2->val + carry)%10);
            carry = (l1->val + l2->val + carry)/10;
            if(head == NULL){
                head = newNode;
                pre = head;
            }else{
                pre->next = newNode;
                pre = newNode;
            }
            l1 = l1->next;
            l2 = l2->next;
        }
        
        while(l1){
            ListNode * newNode = new ListNode((l1->val + carry)%10);
            carry = (l1->val + carry)/10;
            pre->next = newNode;
            pre = newNode;
            l1 = l1->next;
        }
        
        while(l2){
            ListNode * newNode = new ListNode((l2->val + carry)%10);
            carry = (l2->val + carry)/10;
            pre->next = newNode;
            pre = newNode;
            l2 = l2->next;
        }
        
        if(carry > 0){
            ListNode * newNode = new ListNode(carry);
            pre->next = newNode;
            pre = newNode;
        }
                
        return head;
    }
};

【中級算法】6.兩數相加