LeetCode Add Two Numbers

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

题意：将两个用链表逆序存储的数进行加法运算，并将结果也用链表逆序存储。如上例的342 + 564 = 807.

题解：掌握链表的头插法和尾插法，从左往右每个节点进行加法运行，进位带入下个个节点运算。用尾插法保存结果数据。要注意节点的空判断，以防出现空指针，当计算最后的进位不为0时，要再加入一个节点存放进位。

class ListNode {
      int val;
      ListNode next;
      ListNode(int x) { val = x; }
 }

public class AddTwoNumbers {
    
    public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode res = new ListNode(0);
        ListNode r = res;
        int x = l1.val;
        int y = l2.val;
        res.val = ( x + y ) % 10;
        int ten = ( x + y ) / 10;
        l1 = l1.next;
        l2 = l2.next;
        
        while(l1 != null || l2 !=null ){
            // 防止出现空指针
            if(l1 == null){
                l1 = new ListNode(0);
                x = 0;
            }else{
                x = l1.val;
            }
            
            if(l2 == null){
                l2 = new ListNode(0);
                y = 0;
            }else{
                y = l2.val;
            }
            
            int single = ( x + y + ten ) % 10;
            ListNode tmp = new ListNode(single);
            r.next = tmp;
            r = tmp;
            ten = ( x + y + ten ) / 10;
            l1 = l1.next;
            l2 = l2.next;
        }
        //存放最后的进位
        if(ten != 0){
            r.next = new ListNode(ten);
        }
        return res;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int a[] = {5};
        int b[] = {5};
        
        ListNode la = new ListNode(0);
        la.val = a[0];
        for(int i = a.length -1 ;i>0;i--){
            ListNode next = new ListNode(a[i]);    
            next.next = la.next;
            la.next = next;
        }
        
        ListNode lb = new ListNode(0);
        lb.val = b[0];
        
        for(int i = b.length -1 ;i > 0 ;i--){
            ListNode next = new ListNode(b[i]);    
            next.next = lb.next;
            lb.next = next;
        }
        
        ListNode c = addTwoNumbers(la,lb);
        
        while( c != null){
            System.out.print(c.val);
            if(c.next != null){
                System.out.print("-->");
            }
            c = c.next;
        }
    }
}