如何使用回溯搜索解决问题？

def solveNQueens(n):
    def is_not_under_attack(row, col):
        return not (cols[col] or hills[row - col] or dales[row + col])

    def place_queen(row, col):
        queens.add((row, col))
        cols[col] = 1
        hills[row - col] = 1
        dales[row + col] = 1

    def remove_queen(row, col):
        queens.remove((row, col))
        cols[col] = 0
        hills[row - col] = 0
        dales[row + col] = 0

    def backtrack(row = 0):
        for col in range(n):
            if is_not_under_attack(row, col):
                place_queen(row, col)
                if row + 1 == n:
                    output.append(queens.copy())
                else:
                    backtrack(row + 1)
                remove_queen(row, col)

    cols = [0] * n
    hills = [0] * (2 * n - 1)
    dales = [0] * (2 * n - 1)
    queens = set()
    output = []
    backtrack()
    return output

# 打印解决方案
solutions = solveNQueens(8)
for solution in solutions:
    board = [['.' for _ in range(8)] for _ in range(8)]
    for row, col in solution:
        board[row][col] = 'Q'
    for row in board:
        print(' '.join(row))
    print()