地铁线路最短路径（实现）

主要功能

提供一副地铁线路图，计算指定两站之间最短（最少经过站数）乘车路线；输出指定地铁线路的所有站点。以北京地铁为例，地铁线路信息保存在地铁线路信息.txt中，格式如下：

地铁线路总数
线路名1 站名1 站名2 站名3 ...
线路名2 站名1 站名2 站名3 ...
线路名3 站名1 站名2 站名3 ......

实现语言

python

实现算法

BFS

类职责划分

ReadFile(file_name)：读入地铁信息

build_subway(**lines)：构建图结构

shortest_path(start, goal):用BFS找最短路径

核心代码

数据读入：

def ReadFile(file_name):
    data=dict()
    with open(file_name, "rt", encoding="utf-8") as file:
        n=int(file.readline())
        for i in range(n):
            value=""
            line=file.readline().split()
            for i in line[1:]:
                value=value+i+" "
            data[line[0]]=value
    return data

构建图结构：

def BuildMap(**lines):
    for key in lines.keys():
        value = lines[key]
        lines[key] = value.split()
    stations = set()
    for key in lines.keys():
        stations.update(set(lines[key]))
    system = {}
    for station in stations:
        next_station = {}
        for key in lines:
            if station in lines[key]:
                line = lines[key]
                idx = line.index(station)
                if idx == 0:
                    next_station[line[1]] = key
                elif idx == len(line)-1:
                    next_station[line[idx-1]]=key
                else:
                    next_station[line[idx-1]] = key
                    next_station[line[idx+1]] = key
        system[station] = next_station
    return system

处理环路：

def DealCircle(BJ_Subway): #发现环路时将其头尾相连
    
    BJ_Subway[u'西直门'][u'积水潭'] = '二号线'
    BJ_Subway[u'积水潭'][u'西直门'] = '二号线'
    BJ_Subway[u'劲松'][u'潘家园'] = '十号线'
    BJ_Subway[u'潘家园'][u'劲松'] = '十号线'
    return BJ_Subway
fin_subway = DealCircle(tmp)

用BFS找最短路径：

def FindPath(start, goal):
    if start == goal:
        return [start]
    explored = set() 
    queue = [ [start, ('',0)]] 
    while queue:
        path = queue.pop(0)
        s = path[-2]
        linenum,changetimes = path[-1]#增加判断换乘的元组，linenum表示上一次的线路，changetimes表示
        if s == goal:
            return path
        for state, action in fin_subway[s].items():
            if state not in explored:
                linechange = changetimes
                explored.add(state)
                if linenum != action:
                    linechange += 1
                path2 = path[:-1] + [action, state, (action, linechange)]
                queue.append(path2)
                queue.sort(key=lambda path:path[-1][-1])
    return []