LeetCode刷题（22）

N皇后问题，回溯法。

class Solution(object):
    def solveNQueens(self, n):
        """
        :type n: int
        :rtype: List[List[str]]
        """        
        if n == 1:
            return [["Q"]]
        elif n < 4:
            return []

        x = [0] * (n + 1)
        res = []
        k = 1
        while k>=1:
            x[k] = x[k] + 1
            while x[k]<=n and not valid(x, k):
                x[k] = x[k] + 1
            if x[k] <= n and k == n:
                tmp = []
                for i in range(1, n+1):
                    tmp_s = ""
                    for j in range(n):
                        if x[i] - 1 == j:
                            tmp_s += "Q"
                        else :
                            tmp_s += "."
                    tmp += [tmp_s]
                res += [tmp]
            elif x[k] <= n and k < n :
                k += 1
            else:
                x[k] = 0
                k -= 1

        return res

def valid(x, k):
    for i in range(1, k):
        if x[i] == x[k] or (k - i) == abs(x[k] - x[i]):
            return False
    return True