使用ABT（The asynchronous backtracking algorithm）算法求解四皇后（4-queens）问题，使用python

以下是使用ABT算法求解四皇后问题的Python代码：

def abt_search(csp):
    return abt_recursive(csp, {})

def abt_recursive(csp, assignment):
    if len(assignment) == len(csp.variables):
        return assignment
    var = select_unassigned_variable(csp, assignment)
    for value in order_domain_values(var, assignment, csp):
        if is_consistent(var, value, assignment, csp):
            csp.assign(var, value)
            inferences = {}
            for neighbor in csp.neighbors(var):
                if neighbor not in assignment:
                    for removed_value in csp.curr_domain_values(neighbor):
                        if not csp.constraints(var, value, neighbor, removed_value):
                            inferences[neighbor] = removed_value
                            csp.eliminate(neighbor, removed_value)
            result = abt_recursive(csp, assignment)
            if result is not None:
                return result
            csp.unassign(var)
            for neighbor, removed_value in inferences.items():
                csp.revise(neighbor, removed_value)
    return None

def select_unassigned_variable(csp, assignment):
    for var in csp.variables:
        if var not in assignment:
            return var

def order_domain_values(var, assignment, csp):
    return csp.curr_domain_values(var)

def is_consistent(var, value, assignment, csp):
    for neighbor in csp.neighbors(var):
        if neighbor in assignment:
            if not csp.constraints(var, value, neighbor, assignment[neighbor]):
                return False
    return True

class CSP:
    def __init__(self, variables, domains, constraints):
        self.variables = variables
        self.domains = domains
        self.constraints = constraints
        self.unassigned_vars = set(variables)
        self.curr_domains = {v: set(domains) for v in variables}
        
    def assign(self, var, value):
        self.unassigned_vars.remove(var)
        self.curr_domains[var] = set([value])
        
    def unassign(self, var):
        self.unassigned_vars.add(var)
        self.curr_domains[var] = set(self.domains)
        
    def eliminate(self, var, value):
        self.curr_domains[var].remove(value)
        
    def revise(self, var, value):
        self.curr_domains[var].add(value)
        
    def curr_domain_values(self, var):
        return self.curr_domains[var]
    
    def neighbors(self, var):
        return set(self.variables) - set([var])
    
    def is_complete(self, assignment):
        return len(assignment) == len(self.variables)
    
def four_queens_csp():
    variables = ['Q1', 'Q2', 'Q3', 'Q4']
    domains = range(1, 5)
    constraints = lambda Q1, Q2, Q3, Q4: (Q1 != Q2 and Q1 != Q3 and Q1 != Q4 and Q2 != Q3 and Q2 != Q4 and Q3 != Q4 and abs(Q1 - Q2) != 1 and abs(Q1 - Q3) != 2 and abs(Q1 - Q4) != 3 and abs(Q2 - Q3) != 1 and abs(Q2 - Q4) != 2 and abs(Q3 - Q4) != 1)
    return CSP(variables, domains, constraints)

if __name__ == '__main__':
    csp = four_queens_csp()
    solution = abt_search(csp)
    if solution is not None:
        print(solution)
    else:
        print('No solution found')

该代码定义了一个CSP类，其中包含“assign”、“unassign”、“eliminate”和“revise”等方法，用于实现ABT算法。它还定义了一个“four_queens_csp”函数，用于创建四皇后问题的CSP实例。最后，它调用“abt_search”函数来解决问题，并输出解决方案。