86.py

'''
	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.

	Example:

	Input: head = 1->4->3->2->5->2, x = 3
	Output: 1->2->2->4->3->5
'''

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def partition(self, head, x):
        """
        :type head: ListNode
        :type x: int
        :rtype: ListNode
        """
        if not head or not head.next:
        	return head

        left, right = ListNode(0), ListNode(0)
        leftPtr, rightPtr = left, right

        while head:
        	if head.val < x:
        		leftPtr.next = ListNode(head.val)
        		leftPtr = leftPtr.next
        	else:
        		rightPtr.next = ListNode(head.val)
        		rightPtr = rightPtr.next
        	head = head.next

        if not left.next:
        	return right.next
        elif not right.next:
        	return left.next
        else:
        	leftPtr.next = right.next
        	return left.next
# Time: O(N)
# Space: O(N)