# Problem 86: Partition List

> <https://leetcode.com/problems/partition-list/>

**类比**: Partion Array 这道题

> <http://www.lintcode.com/en/problem/partition-array/>

## 思路

* 创建一个left LinkedList，一个right LinkedList。
* 遇到小的挂left上，遇到大的挂right上
* 最后合并 left 和 right

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode partition(ListNode head, int x) {
        if (head == null) {
            return head;
        }

        ListNode leftDummy = new ListNode(0);
        ListNode rightDummy = new ListNode(0);
        ListNode left = leftDummy;
        ListNode right = rightDummy;

        while (head != null) {
            if (head.val < x) {
                left.next = head;
                left = head;
            } else {
                right.next = head;
                right = head;
            }
            head = head.next;
        }

        right.next = null;
        left.next = rightDummy.next;

        return leftDummy.next;
    }
}
```

## 易错点

1. 两个指针，left用来探路，leftDummy用来标记“头”

   ```java
   ListNode left = leftDummy;
   ListNode right = rightDummy;
   ```
2. LinkedList的循环

   ```java
   while (head != null) {
    执行语句;
    head = head.next;
   }
   ```

   其中，`head = head.next;`作为增加变量使用的。
3. 掐头去尾进行拼接

   ```java
   right.next = null;
   left.next = rightDummy.next;
   ```


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