多维列表元素异常变更：N皇后回溯算法调试求助

我正在实现N皇后问题的解决方案，编写代码计算n×n棋盘上放置n个皇后的可行方案数。以下是我的代码：

n = int(input())
piony = [[False for i in range(n)] for j in range(n)]
skosy_lp = [[False for i in range(n)] for j in range(n)]
skosy_pl = [[False for i in range(n)] for j in range(n)]
figury = [-1] * n
counter = 0
layer = 0
# list creating etc. piony list is containing vertical lines occupied by queens, skosy_lp
# is containing lines oblique from left to right, skosy_pl - from right to left.
# this is a backtracking algorithm
# figury list is containing positions of queens. -1 means that the queen isn't placed
while True:
    if layer == -1:
        break
    piony[layer] = piony[layer-1]
    skosy_pl[layer] = skosy_pl[layer-1]
    skosy_pl[layer].append(False)
    skosy_pl[layer].pop(0)
    skosy_lp[layer] = skosy_lp[layer-1]
    skosy_lp[layer].insert(0, False)
    skosy_lp[layer].pop(n)
    figury[layer]+=1
    while figury[layer] < n and (piony[layer][figury[layer]] or skosy_lp[layer][figury[layer]]or skosy_lp[layer][figury[layer]]):
        figury[layer] +=1
    if figury[layer] == n:
        figury[layer] = -1
        piony[layer] = [False] * n
        skosy_pl[layer] = [False] * n
        skosy_lp[layer] = [False] * n
        layer-=1
    else:
        if layer == n-1:
            counter+=1
            layer-=1
        else:
            print(piony)
            # I think there is a bug
            piony[layer][figury[layer]] = True
            skosy_pl[layer][figury[layer]] = True
            skosy_lp[layer][figury[layer]] = True
            layer+=1

调试时发现，当layer=0且figury[layer]=0时，执行piony[layer][figury[layer]] = True会同时修改piony[0][0]和piony[-1][0]，请问这是什么原因？如何修复？

问题原因 #

核心是列表引用赋值导致的对象共享：

• 当layer=0时，layer-1等于-1，对应piony列表的最后一个元素（Python中负数索引表示从末尾倒数）。
• 执行piony[layer] = piony[layer-1]时，只是把piony[-1]的内存引用赋值给piony[0]，两者指向同一个列表对象。此时修改piony[0][0]，本质是修改这个共享的列表对象，所以piony[-1][0]也会同步变化。
• 同理，skosy_pl和skosy_lp的赋值逻辑也存在相同问题，会导致不同layer的列表互相影响。

修复方案 #

把引用赋值改为拷贝赋值，创建独立的列表副本，避免共享内存对象：

• 将piony[layer] = piony[layer-1]修改为：

  piony[layer] = piony[layer-1].copy()
  # 或使用切片语法：piony[layer] = piony[layer-1][:]
  

• 对skosy_pl和skosy_lp做同样修改：

  skosy_pl[layer] = skosy_pl[layer-1].copy()
  skosy_lp[layer] = skosy_lp[layer-1].copy()
  

• 这样每个layer对应的列表都是独立的新对象，修改其中一个不会影响其他layer的列表。

内容的提问来源于stack exchange，提问作者Filiput135