Problem:
The goal of 8 Queens Problem is to put eight queens on a chess-board such that none of them threatens any of others. A queen threatens the squares in the same row, in the same column, or on the same diagonals as shown in the following figure.      For a given chess board where $k$ queens are already placed, find the solution of the 8 queens problem.

Incorrect submission (verdict: Wrong Answer):
from copy import deepcopy, copy
class QueenMAP():
    __slots__ = ["yoko", "tate", "naname1", "naname2", "MAP"]
    def __init__(self):
        self.yoko = set()
        self.tate = set()
        self.naname1 = set()
        self.naname2 = set()
        self.MAP = [["."] * 8 for _ in range(8)]
    def add(self, y, x):
        self.MAP[y][x] = "Q"
        self.yoko.add(y)
        self.tate.add(x)
        self.naname1.add(y - x)
        self.naname2.add(x + y)
    def check(self, y, x):
        if x in self.tate or (y - x) in self.naname1 or (x - y) in self.naname2:
            return False
        return True
    def allcheck(self):
        for i in range(8):
            if not "Q" in self.MAP[i]:
                return False
        return True
def MAIN():
    f = lambda M: "\n".join("".join(map(str, m)) for m in M)
    QM = QueenMAP()
    n = int(input())
    for _ in range(n):
        a, b = map(int, input().split())
        QM.add(a, b)
    dp = [(deepcopy(QM), n)]
    while dp:
        Q, cnt = dp.pop()
        if cnt == 8:
            if Q.allcheck():
                print(f(Q.MAP))
                break
            continue
        cnt += 1
        for i in range(8):
            if i in Q.yoko:
                continue
            for j in range(8):
                if Q.check(i, j):
                    CQ = deepcopy(Q)
                    CQ.add(i, j)
                    dp.append((CQ, cnt))
MAIN()



Fixed, accepted submission:
from itertools import product
from copy import deepcopy, copy
class QueenMAP():
    __slots__ = ["yoko", "tate", "naname1", "naname2", "MAP"]
    def __init__(self):
        self.yoko = set()
        self.tate = set()
        self.naname1 = set()
        self.naname2 = set()
        self.MAP = [["."] * 8 for _ in range(8)]
    def add(self, y, x):
        self.MAP[y][x] = "Q"
        self.yoko.add(y)
        self.tate.add(x)
        self.naname1.add(y - x)
        self.naname2.add(x + y)
    def check(self, y, x):
        if y in self.yoko or x in self.tate or (y - x) in self.naname1 or (x + y) in self.naname2:
            return False
        return True
    def allcheck(self):
        for i in range(8):
            if not "Q" in self.MAP[i]:
                return False
        return True
    def show(self):
        print("\n".join("".join(m) for m in self.MAP))
def MAIN():
    f = lambda M: "\n".join("".join(m) for m in M)
    QM = QueenMAP()
    n = int(input())
    for _ in range(n):
        a, b = map(int, input().split())
        QM.add(a, b)
    dp = [(deepcopy(QM), n)]
    while dp:
        Q, cnt = dp.pop()
        if cnt == 8:
            if Q.allcheck():
                Q.show()
                break
            continue
        cnt += 1
        for i, j in product(range(8), repeat=2):
            if Q.check(i, j):
                CQ = deepcopy(Q)
                CQ.add(i, j)
                dp.append((CQ, cnt))
MAIN()