我已经编写了一个python代码来解决使用python中的递归dfs的传教士和食人族问题。但是我一直收到这个错误： RecursionError：超出最大递归深度
我不知道该怎么办，而且我已经坚持了很久。 任何帮助或建议都将为我节省生命。谢谢。 这是代码：
class State(object):
#left = 1
#right = 0 for boat
def __init__(self, missionaries, cannibals, boat):
self.missionaries = missionaries
self.cannibals = cannibals
self.boat = boat
#def __str__(self):
# return "%s, %s %s %s" % (self.by_move, self.missionaries, self.cannibals, self.boat)
def is_valid(self):
if self.missionaries < 0 or self.missionaries > 3:
return False
if self.cannibals < 0 or self.cannibals > 3:
return False
if self.boat > 1 or self.boat < 0:
return False
if self.missionaries < self.cannibals and self.missionaries > 0:
return False
# Check for the other side
if self.missionaries > self.cannibals and self.missionaries < 3:
return False
return True
def is_goal(self):
return self.missionaries == 0 and self.cannibals == 0 and self.boat == 0
def new_states(self):
op = -1 # Subtract
boat_move = "from left shore to right"
if self.boat == 0:
op = 1 # Add
boat_move = "from right shore to left"
for x in range(3):
for y in range(3):
by_move = "Move %s missionaries and %s cannibals %s" % (x, y, boat_move)
new_state = State(self.missionaries + op * x, self.cannibals + op * y, self.boat + op * 1)
if x + y >= 1 and x + y <= 2 and new_state.is_valid():
yield new_state
class Node(object):
def __init__(self, parent, state, depth):
self.parent = parent
self.state = state
self.depth = depth
def children(self):
for state in self.state.new_states():
yield Node(parent=self, state=state, depth=self.depth + 1)
def extract_solution(self):
print
"Extracting soln"
solution = []
node = self
solution.append(node)
while node.parent is not None:
solution.append(node.parent)
node = node.parent
solution.reverse()
return solution
def dfs(root,visited,sol = None):
if root in visited:
return
if root is None:
return
visited.append(root)
if root.state.is_goal():
sol = root
return
for child in root.children():
if child not in visited:
dfs(child,visited,sol)
def main():
initial_state = State(3,3,1)
root = Node(parent = None, state = initial_state,depth = 0)
visited = []
sol = Node(parent = None, state = initial_state,depth = 0)
dfs(root,visited,sol)
ans = sol.extract_solution()
print(ans)
if __name__ == '__main__':
main()
答案 0 :(得分：0)
有两个问题：
1：您的列表已访问过＆＃39;没有正确跟踪所有州。这可以通过访问一个全局变量（通过将其置于def main（）之前，如最终解决方案中所做的那样）轻松修复。
2：该计划正在寻找无法帮助的可能性（例如：来回带来同一个人），这个
if root in visited:
return
if root is None:
return
没有解决这个问题，因为它永远不会是同一个根对象（即使root.state.mission，canibals和boat是相同的值），所以我使用字典对象改变了这个： / p>
if root is None:
return
state = str(root.state.missionaries) + ',' + str(root.state.cannibals) + ',' + str(root.state.boat)
if state in routes:
if routes[state] < root.depth:
return
else:
routes[state] = root.depth
else:
routes[state] = root.depth
visited.append(root)
这导致以下代码（它返回一个答案，我不确定它是否正确，因为我不知道传教士和食人族的问题）
class State(object):
#left = 1
#right = 0 for boat
def __init__(self, missionaries, cannibals, boat):
self.missionaries = missionaries
self.cannibals = cannibals
self.boat = boat
#def __str__(self):
# return "%s, %s %s %s" % (self.by_move, self.missionaries, self.cannibals, self.boat)
def is_valid(self):
if self.missionaries < 0 or self.missionaries > 3:
return False
if self.cannibals < 0 or self.cannibals > 3:
return False
if self.boat > 1 or self.boat < 0:
return False
if self.missionaries < self.cannibals and self.missionaries > 0:
return False
# Check for the other side
if self.missionaries > self.cannibals and self.missionaries < 3:
return False
return True
def is_goal(self):
return self.missionaries == 0 and self.cannibals == 0 and self.boat == 0
def new_states(self):
op = -1 # Subtract
boat_move = "from left shore to right"
if self.boat == 0:
op = 1 # Add
boat_move = "from right shore to left"
for x in range(3):
for y in range(3):
by_move = "Move %s missionaries and %s cannibals %s" % (x, y, boat_move)
new_state = State(self.missionaries + op * x, self.cannibals + op * y, self.boat + op * 1)
if x + y >= 1 and x + y <= 2 and new_state.is_valid():
yield new_state
class Node(object):
def __init__(self, parent, state, depth):
self.parent = parent
self.state = state
self.depth = depth
def children(self):
for state in self.state.new_states():
yield Node(parent=self, state=state, depth=self.depth + 1)
def extract_solution(self):
print "Extracting soln"
solution = []
node = self
solution.append(node)
while node.parent is not None:
solution.append(node.parent)
node = node.parent
solution.reverse()
return solution
def dfs(root,sol = None):
if root is None:
return
state = str(root.state.missionaries) + ',' + str(root.state.cannibals) + ',' + str(root.state.boat)
if state in routes:
if routes[state] < root.depth:
return
else:
routes[state] = root.depth
else:
routes[state] = root.depth
visited.append(root)
if root.state.is_goal():
sol = root
return
for child in root.children():
if child not in visited:
dfs(child,sol)
visited = []
routes = {}
def main():
initial_state = State(3,3,1)
root = Node(parent = None, state = initial_state,depth = 0)
sol = Node(parent = None, state = initial_state,depth = 0)
dfs(root,sol)
ans = sol.extract_solution()
print(ans)
if __name__ == '__main__':
main()
PS。正如@PM 2Ring所说，下次：请在提问时修改你的缩进，这使得阅读代码更容易理解。您可以通过选择所有代码，向所有选定行添加选项卡然后复制它来完成此操作。在粘贴之前，请确保空行。 ：）