leetcode 88. merge two sorted lists，关于python list赋值，与引用

本文链接：https://blog.youkuaiyun.com/weixin_47426299/article/details/144228176

问题描述

Merge Sorted Array
You are given two integer arrays nums1 and nums2, sorted in non-decreasing order, and two integers m and n, representing the number of elements in nums1 and nums2 respectively.
Merge nums1 and nums2 into a single array sorted in non-decreasing order.
The final sorted array should not be returned by the function, but instead be stored inside the array nums1. To accommodate this, nums1 has a length of m + n, where the first m elements denote the elements that should be merged, and the last n elements are set to 0 and should be ignored. nums2 has a length of n.

一个标准回答

class Solution(object):
    def merge(self, nums1, m, nums2, n):

      for j in range(n):
          nums1[m+j] = nums2[j]
      nums1.sort()

非常好理解。

我的一个错误回答

class Solution(object):
    def merge(self, nums1, m, nums2, n):
        """
        :type nums1: List[int]
        :type m: int
        :type nums2: List[int]
        :type n: int
        :rtype: None Do not return anything, modify nums1 in-place instead.
        """
        nums1_temp = nums1[:m]
        print(nums1_temp)
        merge_unsort = nums1_temp+nums2
        print(merge_unsort)
        merge_unsort.sort()
        print(merge_unsort)
        nums1 = merge_unsort
        print(nums1)
        return nums1

这里有两个地方要注意，一是list.sort()和sorted(list)的区别。前者没有返回值，会直接改变list本身的值。
二是list的赋值，并不能通过=号直接给，=这里是一个引用的作用，nums1指向了Merge_unsort,并不是"The final sorted array should not be returned by the function, but instead be stored inside the array nums1". nums1本身的值必须改变。所以如果把最后改成：

nums1[:] = merge_unsort

也是可以的。

如何确定是本身的值变了，还是引用变了呢

可以通过打印地址，id(list)来确定。

print(id(nums1))
nums1 = merge_unsort
print(id(merge_unsort))
print(id(nums1))

#output
139976362657504
139976362709232
139976362709232

可以看到nums1在=之后指向merge_unsort的地址了。

class Solution(object):
    def merge(self, nums1, m, nums2, n):
      print(id(nums1))
      for j in range(n):
          nums1[m+j] = nums2[j]
      nums1.sort()
      print(id(nums1))
# output
140265616881456
140265616881456