[一起来刷leetcode吧][28]--No.51 N-Queens

这是leetcode的第51题–N-Queens

  题目

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 all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens’ placement, where ‘Q’ and ‘.’ both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[
 [".Q..",  // Solution 1
  "...Q",
  "Q...",
  "..Q."],

 ["..Q.",  // Solution 2
  "Q...",
  "...Q",
  ".Q.."]
]

  思路
利用递归，依次摆放第i个皇后，注意要求的返回结果是字符串，imutable，不可变，所以先用列表，然后join

  

show me the code

class Solution(object):
    def solveNQueens(self, n):
        """
        :type n: int
        :rtype: List[List[str]]
        """
        rst = []
        nums = [0]*n
        def abs(x):
            if x > 0:
                return x
            return -x
        def find(i):
            if i == n:
                tmp = []
                for count,index in enumerate(nums):
                    s = ['.']*n
                    s[index] = 'Q'
                    tmp.append(''.join(s))
                rst.append(tmp)
                return

            for j in range(0,n):
                ok = True
                for index,k in zip(range(i),nums):
                    if abs(index-i) == abs(j-k) or k == j:
                            ok = False
                            break
                if not ok:
                    continue
                nums[i] = j
                find(i + 1)

        find(0)
        return rst