以给定值为基分割链表

给定一个值，将链表小于该值得结点放到左边，大于该值得放到右边

例如：

Given 1->4->3->2->5->2 and x = 3,
return 1->2->2->4->3->5.

思路：分别将值大于小于给定值得结点生成另外两个链表，最后再将这两个链表相连

代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* partition(ListNode *&head, int x) {
        ListNode *list1 = new ListNode(0);
        ListNode *list2 = new ListNode(0);
        ListNode *before = list1, *after = list2;
        while(head)
        {
            if(head->val < x)
            {
                before->next = head;
                before = before->next;
            }
            else
            {
                after->next = head;
                after = after->next;
            }
            head = head->next;
        }
        after->next = NULL;
        before->next = list2->next;
        return list1->next;
    }
};