Partition List

Question

• leetcode: Partition List | LeetCode OJ
• lintcode: (96) Partition List

Problem Statement

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.

题解

此题出自 CTCI 题 2.4，依据题意，是要根据值x对链表进行分割操作，具体是指将所有小于x的节点放到不小于x的节点之前，咋一看和快速排序的分割有些类似，但是这个题的不同之处在于只要求将小于x的节点放到前面，而并不要求对元素进行排序。

这种分割的题使用两路指针即可轻松解决。左边指针指向小于x的节点，右边指针指向不小于x的节点。由于左右头节点不确定，我们可以使用两个dummy节点。

Python

"""
Definition of ListNode
class ListNode(object):

    def __init__(self, val, next=None):
        self.val = val
        self.next = next
"""
class Solution:
    """
    @param head: The first node of linked list.
    @param x: an integer
    @return: a ListNode
    """
    def partition(self, head, x):
        if head is None:
            return None

        leftDummy = ListNode(0)
        left = leftDummy
        rightDummy = ListNode(0)
        right = rightDummy
        node = head
        while node is not None:
            if node.val < x:
                left.next = node
                left = left.next
            else:
                right.next = node
                right = right.next
            node = node.next
        # post-processing
        right.next = None
        left.next = rightDummy.next

        return leftDummy.next

• reference

"""
Definition of ListNode
class ListNode(object):

    def __init__(self, val, next=None):
        self.val = val
        self.next = next
"""
class Solution:
    """
    @param head: The first node of linked list.
    @param x: an integer
    @return: a ListNode 
    """
    def partition(self, head, x):
        # write your code here
        if head is None:
            return head
        aHead, bHead = ListNode(0), ListNode(0)
        aTail, bTail = aHead, bHead
        while head is not None:
            if head.val < x:
                aTail.next = head
                aTail = aTail.next
            else:
                bTail.next = head
                bTail = bTail.next
            head = head.next
        bTail.next = None
        aTail.next = bHead.next
        return aHead.next

源码分析

1. 异常处理
2. 引入左右两个dummy节点及left和right左右尾指针
3. 遍历原链表
4. 处理右链表，置right->next为空(否则如果不为尾节点则会报错，处理链表时 以 null 为判断)，将右链表的头部链接到左链表尾指针的next，返回左链表的头部

复杂度分析

遍历链表一次，时间复杂度近似为 $$O(n)$$, 使用了两个 dummy 节点及中间变量，空间复杂度近似为 $$O(1)$$.