leetcode [Add Two Numbers]

解法一：

/** 
 * Definition for singly-linked list. 
 * public class ListNode { 
 *     int val; 
 *     ListNode next; 
 *     ListNode(int x) { val = x; } 
 * } 
 */  
public class Solution {  
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {  
        ListNode res = new ListNode(0);  
        ListNode tempNode;  
        ListNode shift = res;//用shift来代替res移位  
        int temp = 0;  
        int carry = 0;  
        //共同的位  
        while(l1 != null && l2 != null){  
            temp = (l1.val + l2.val + carry) % 10;  
            carry = (l1.val + l2.val + carry) / 10;  
            tempNode = new ListNode(temp);  
            shift.next = tempNode;//返回结果时记得将res右移一位  
            shift = tempNode;  
            l1 = l1.next;//记得将l1与l2往后移  
            l2 = l2.next;  
        }  
        //位数不同的处理  
        while(l1 != null){  
            temp = (l1.val + carry) % 10;  
            carry = (l1.val + carry) / 10;  
            tempNode = new ListNode(temp);  
            shift.next = tempNode;  
            shift = tempNode;  
            l1 = l1.next;//记得将l1往后移  
        }  
        while(l2 != null){  
            temp = (l2.val + carry) % 10;  
            carry = (l2.val + carry) / 10;  
            tempNode = new ListNode(temp);  
            shift.next = tempNode;  
            shift = tempNode;  
            l2 = l2.next;//记得将l2往后移  
        }  
        //处理额外的进位  
        if(carry != 0){  
            tempNode = new ListNode(carry);//是carry不是temp  
            shift.next = tempNode;  
        }  
        res = res.next;  
        return res;  
    }  
}

 同样的方法，更简洁的代码：

    public class Solution {  
        public ListNode addTwoNumbers(ListNode l1, ListNode l2) {  
            ListNode c1 = l1;  
            ListNode c2 = l2;  
            ListNode sentinel = new ListNode(0);  
            ListNode d = sentinel;  
            int sum = 0;  
            while (c1 != null || c2 != null) {  
                sum /= 10;  
                if (c1 != null) {  
                    sum += c1.val;  
                    c1 = c1.next;  
                }  
                if (c2 != null) {  
                    sum += c2.val;  
                    c2 = c2.next;  
                }  
                d.next = new ListNode(sum % 10);  
                d = d.next;  
            }  
            if (sum / 10 == 1)  
                d.next = new ListNode(1);  
            return sentinel.next;  
        }  
    }