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
正常思路是反转链表+1然后再反转回去。 发现有个大神没反转, 找到最后一个非9的节点,然后将这个节点++, 后面所有的节点将9 变成0; 如果这个节点是头节点, 还要看当前节点是头节点而且值=9, 那么要new你一个1 的新节点。