Leetcode: N-Queens @Python

class Solution:
    # @return a list of lists of string
    def under_attack(self,col, queens):
        return col in queens or any(abs(col - x) == len(queens)-i for i,x in enumerate(queens))
            
    def solve(self,n):
        solutions = [[]]
        for row in range(n):
            solutions = (solution+[i]
            for solution in solutions # first for clause is evaluated immediately,
            # so "solutions" is correctly captured
            for i in range(n)
            if not self.under_attack(i, solution))
        return solutions

    
    def solveNQueens(self, n):
        slt=[]
        for j in self.solve(n):
            if len(j)==n:
                slt.append(list('.'*i+'Q'+'.'*(n-1-i) for i in j))
        return slt