2 两树相加

大数相加转化为位相加的思路

方法一：位相加循环

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
       int sum = 0;
        ListNode* result = new ListNode(0);//保留住链表头
        ListNode* temp = result;
        while (l1!=NULL||l2!=NULL||sum){
            l1 = l1!=NULL ? (sum+=l1->val, l1=l1->next) : l1;
            l2 = l2!=NULL ? (sum+=l2->val, l2=l2->next) : l2;
            temp->next = new ListNode(sum%10);
            temp = temp->next;
            sum = sum/10;
        }
        return result->next; 
    }
};

方法二：递归

class Solution {
public:
    int sum = 0;
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
       if(l1==NULL&&l2==NULL&&sum==0)
            return NULL;
        l1 = l1==NULL ? l1 : (sum+=l1->val,l1=l1->next);
        l2 = l2==NULL ? l2 : (sum+=l2->val,l2=l2->next);
        ListNode* res = new ListNode(0);
        res->val = sum%10;
        sum = sum/10;
        res->next = addTwoNumbers(l1,l2);
        return res;
    }
};