LeetCode-148. Sort List

148. Sort List

Sort a linked list in O(n log n) time using constant space complexity.

Solution

利用归并排序的思想

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

class Solution(object):
    def sortList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head or not head.next:
            return head
        p1 = p2 = pre = head
        while p2 and p2.next:
            pre = p1
            p1 = p1.next
            p2 = p2.next.next
        pre.next = None
        h1 = self.sortList(head)
        h2 = self.sortList(p1)
        return self.merge(h1, h2)
        
    def merge(self, h1, h2):
        p = ListNode(None)
        l = p
        while h1 and h2:
            if h1.val < h2.val:
                p.next = h1
                h1 = h1.next
            else:
                p.next = h2
                h2 = h2.next
            p = p.next
        p.next = h1 or h2
        return l.next
        
    """
    # 会出现超过最大递归次数的错误。RuntimeError: maximum recursion depth exceeded
    def merge(self, h1, h2):
        if not h1:
            return h2
        if not h2:
            return h1
        if h1.val < h2.val:
            h1.next = self.merge(h1.next, h2)
            return h1
        else:
            h2.next = self.merge(h1, h2.next)
            return h2
    """