4 皇后有几个解

4×4 的棋盘上放 4 个互不攻击的皇后。运行下面这段程序：

def ok(col, c):
    r = len(col)
    for i in range(r):
        if col[i] == c or abs(col[i] - c) == r - i:
            return False
    return True

def queens(n):
    st = {"c": 0, "v": 0}
    col = []
    def dfs():
        st["v"] += 1
        if len(col) == n:
            st["c"] += 1
            return
        for c in range(n):
            if not ok(col, c):
                continue
            col.append(c)
            dfs()
            col.pop()
    dfs()
    return st["c"], st["v"]

print(queens(4)[0])