109. 排序鏈表

解題思路

常數集空間復雜度：將其轉化為數組；

時間復雜度為O(nlogn)：用歸並排序；

實現

# 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:
            return None
 
     # 轉化為數組
        result = []
        head_tmp = head
        while head_tmp:
            result.append(head_tmp.val)
            head_tmp = head_tmp.next
        
        # 歸並排序
        result = self.merge_sort(result)
        # 將排序後的結果重新填充到鏈表中
        head_tmp
 
 = head
        for i in range(len(result)):
            head_tmp.val = result[i]
            head_tmp = head_tmp.next
        
        return head
        
    def merge(self, left, right):
        i, j = 0, 0
        left_len, right_len = len(left), len(right)
        result = []
        while i < left_len and
 
 j < right_len:
            if left[i] < right[j]:
                result.append(left[i])
                i += 1
            else:
                result.append(right[j])
                j += 1
        result += left[i:]
        result += right[j:]
        return result
​
    def merge_sort(self, lists):
        lists_len = len(lists)
        if lists_len <= 1:
            return lists
​
        center = lists_len / 2
        left = self.merge_sort(lists[:center])
        right = self.merge_sort(lists[center:])
        return self.merge(left, right)

109. 排序鏈表