Partition List

Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x.

You should preserve the original relative order of the nodes in each of the two partitions.

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

分析：头结点的应用对本程序至关重要，降低了编程难度；

code:

/**
 * 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) {
        if(head==NULL)
            return head;
            
        ListNode lt(-1);
        ListNode rt(-1);
        ListNode *lt_next=<
        ListNode *rt_next=&rt;
        
        while(head)
        {
            if(head->val<x)
            {
                lt_next->next=head;
                lt_next=head;
            }
            else
            {
                rt_next->next=head;
                rt_next=head;
            }
            head=head->next;
        }
        
        lt_next->next=rt.next;
        rt_next->next=NULL;
        
        return lt.next;
    }
};