You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

总结：指针操作还是要多练习。

/**
 * 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(l2==NULL) return l1;  
        if(l1==NULL) return l2;  
        
        ListNode *l3=NULL;  
        ListNode *p = l3;  
        int sum;
        int flag=0;  
        while( l1 || l2 ){  
            sum =  flag;  
            if(l1){  
                sum += l1->val;  
                l1 = l1->next;  
            }  
            if(l2){  
                sum += l2->val;  
                l2 = l2->next;  
            }  
            
            flag = sum /10;  
            sum = sum%10;  
              
            if(!l3){  
                l3 = new ListNode(sum);  
                p  = l3;  
            }else{  
                p->next = new ListNode(sum);  
                p = p->next;  
            }  
        }  
        
        if ( flag != 0){  
            p->next = new ListNode(flag);  
        }  
        
        return l3;  
    }  
};

class Solution {
public:
    int climbStairs(int n) {
        int arr[200];
        int i;
        
        arr[0] = 1;
        arr[1] = 2;
        for(i=2;i<n;i++){
            arr[i] = arr[i-1] + arr[i-2];
        }
        
        return arr[n-1];
    }
};