2.4---把链表划分为两部分（CC150）

注意，题目要求要保持两部分的相对顺序，所以，用交换是不行的。

import java.util.HashSet;
import java.util.Set;

class ListNode{
    int val;
    ListNode next;
    ListNode(int x){
        val = x;
    }
}

public class Solution{

    public static void main(String[] args){
        ListNode head = new ListNode(3);
        head.val = 3;
        ListNode node = new ListNode(1);
        node.val = 1;
        head.next = node;
        ListNode tmp = head;
        while(tmp != null){
            System.out.println(tmp.val);
            tmp = tmp.next;
        }
        tmp = partition(head,2);
        System.out.println("delete");

        while(tmp != null){

            System.out.println(tmp.val);
            tmp = tmp.next;
        }
    }
    public static ListNode partition(ListNode head, int x) {
        //思路：用两个链表记录一下两部分，然后合并
        if(null == head)
            return head;
        ListNode greater = new ListNode(0);//小的
        ListNode smaller = new ListNode(0);//>=
        ListNode tmp1 = greater;
        ListNode tmp2 = smaller;

        ListNode tmp = head;
        while(tmp != null)
        {
            if(tmp.val < x)
            {
                ListNode n = new ListNode(tmp.val);
                tmp2.next = n;
                tmp2 = tmp2.next;//error: not write
            }
            else
            {
                ListNode m = new ListNode(tmp.val);
                tmp1.next = m;
                tmp1 = tmp1.next;//error: not write
            }
            tmp = tmp.next;//总是忘记写这一步
        }
        //最后合并
        tmp1.next = null;
        tmp2.next = greater.next;
        return smaller.next;
    }
}