Given a non-negative number represented as a singly linked list of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

Example:

Input:
1->2->3

Output:
1->2->4

分析：
递归做法：
/** 
 * Definition for singly-linked list. 
 * public class ListNode { 
 *     int val; 
 *     ListNode next; 
 *     ListNode(int x) { val = x; } 
 * } 
 */  
public class Solution { 
       public ListNode plusOne(ListNode head) {
        if (head == null)
            return null;
        int carry = helper(head);
        if (carry == 1) {
            ListNode newHead = new ListNode(1);
            newHead.next = head;
            return newHead;
        } else {
            return head;
        }
    }

    public int helper(ListNode head) {
        if (head == null)
            return 1;

        int carry = helper(head.next);
        int value = head.val;
        head.val = (value + carry) % 10;
        carry = (value + carry) / 10;
        return carry;
    }    
}

方法二：

先reverse, 加一，再reverse.

/** 
 * Definition for singly-linked list. 
 * public class ListNode { 
 *     int val; 
 *     ListNode next; 
 *     ListNode(int x) { val = x; } 
 * } 
 */  
public class Solution { 
        private ListNode reverse(ListNode head) {
        ListNode prev = null;
        ListNode current = head;
        ListNode next = null;
        while (current != null) {
            next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }
        return prev;
    }

    public ListNode plusOne(ListNode head) {

        head = reverse(head);
        ListNode newHead = head;
        if (head == null)
            return null;
        int carry = 1;

        ListNode prev = null;
        while (head != null) {
            int value = head.val;
            head.val = (value + carry) % 10;
            carry = (value + carry) / 10;
            prev = head;
            head = head.next;
        }

        if (carry == 1) {
            ListNode end = new ListNode(1);
            prev.next = end;
        }

        return reverse(newHead);
    }
}