# My python solutions to N-Queens I & II, and a question.

• For N-Queens I,

``````def solveNQueens(self, n):
res, myset = [], set()
def dfs(i, tmp, myset):
if i == n:
grid = []
for j in sorted(list(tmp), key = lambda x: x[0]):
grid.append('.'*j[1]+'Q'+'.'*(n-j[1]-1))
res.append(grid)
return
for j in range(n):
if ('a', i) not in myset and ('b', j) not in myset and ('c', i+j) not in myset and ('d', i-j) not in myset:
dfs(i+1, tmp + [(i,j)], myset | set([('a', i), ("b", j), ('c', i+j), ('d', i-j)]))
dfs(0, [], myset)
return res
``````

For N-Queens II,

``````def totalNQueens(self, n):
self.res, myset = 0, set()
def dfs(i, tmp, myset):
if i == n:
self.res += 1
return
for j in range(n):
if ('a', i) not in myset and ('b', j) not in myset and ('c', i+j) not in myset and ('d', i-j) not in myset:
dfs(i+1, tmp + [(i,j)], myset | set([('a', i), ("b", j), ('c', i+j), ('d', i-j)]))
dfs(0, [], myset)
return self.res
``````

Question:

Why in N-Queens I "res" as a list is global, but in N-Queens II "res" as a variable is not global (so I have to use self.res). Is this a general rule in Python?

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