https://leetcode.com/problems/n-queens-ii/description/
The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.
Given an integer n, return the number of distinct solutions to the n-queens puzzle.
Example:
Input: 4 Output: 2 Explanation: There are two distinct solutions to the 4-queens puzzle as shown below. [ [".Q..", // Solution 1 "...Q", "Q...", "..Q."], ["..Q.", // Solution 2 "Q...", "...Q", ".Q.."] ]
class Solution(object):
def isValid(self, board, row, col):
if board[row][col] == 'Q':
return False
n = len(board)
for i in range(n):
if board[i][col] == 'Q':
return False
if board[row][i] == 'Q':
return False
i = 1
while row + i < n or row - i >= 0 or col + i < n or col - i >= 0:
if row + i < n and col + i < n and board[row + i][col + i] == 'Q':
return False
if row + i < n and col - i >= 0 and board[row + i][col - i] == 'Q':
return False
if row - i >= 0 and col + i < n and board[row - i][col + i] == 'Q':
return False
if row - i >= 0 and col - i >= 0 and board[row - i][col - i] == 'Q':
return False
i += 1
return True
def solveNQueens(self, n):
"""
:type n: int
:rtype: List[List[str]]
"""
self.res = 0
board = [['.'] * n for _ in range(n)]
self.solveBoard(board, 0)
return self.res
def solveBoard(self, board, col):
n = len(board)
if col == n:
self.res += 1
return
for i in range(n):
if self.isValid(board, i, col):
board[i][col] = 'Q'
self.solveBoard(board, col + 1)
board[i][col] = '.'