﻿ 一个recursive的深度优先算法问题

# 一个recursive的深度优先算法问题

``````a,b,5
a,c,8
b,d,6
c,d,2
d,e,12
d,f,2
e,g,3
f,g,7``````

``````"""
Graph search to find shortest path between two cities.

A road map is represented as a directed graph.
Edges represent road segments and are labeled with distances.
Nodes represent cities.
Wne use depth first search to find the shortest distance
between two cities. During the depth-first search from
start to end, we compute a shortest known distance to here from start
attribute for each node.  After completion of depth first search,
destination city should be labeled with the shortest distance from
the start city.
"""

import argparse

"""Read a distance table from a named file into a dict
mapping sources to lists of destination, distance pairs.
Args:
map_file: A readable text file in which each line either
begins with #  indicating a comment line or is of the form
from location, to location, distance or time, for example
Minis Tirith,Cair Andros,5
indicating one can travel from Minis Tirith to Cair Andros in 5.0 hours.
Returns:
Dict in which each entry d[from] is a list [to, cost, to, cost, ... ]
where from is a location name, to is a location name, and cost is time
or distance
as a floating point number.  If x,y,z appears in the input file, then
we should have both d[x] contains y,z and d[y] contains x,z, i.e.,
we assume that roads are bi-directional.
"""
connections = dict
for line in map_file:
line = line.strip
if line.startswith"#" or line=="" :
# print"Skipping comment: ", line
continue  ## Discard and go on to next
# print"Processing line: ", line
fields = line.split","
place_from = fields[0]
place_to = fields[1]
cost = floatfields[2]
if not place_from in connections:
connections[place_from] = [ ]
connections[place_from].append place_to, cost
if not place_to in connections:
connections[place_to] = [ ]
connections[place_to].append place_from, cost
return connections

"""Print roads from dict for debugging.
Args:
roads[Chicago] == ["Atlanta", 30.0, "Austin", 25.0]
means that there is a 30.0 mile road from Chicago to Atlanta and a
25.0 mile road from Chicago to Austin.
Returns:
nothing
Effects:
Prints a textual representation of the road connections
"""
printfrom_place + ":"
to_place, dist = hop
print"\t->" + to_place + " " + strdist + ""

"""Depth-first search, which may continue from from_place if dist_so_far
is the shortest distance at which it has yet been reached.
Args:
place: Currently searching from here
dist_so_far:  Distance at which from_place has been reached
this time which may not be the shortest path to from_place
roads:  dict mapping places to lists of hops of the form place, hop-distance
distances: dict mapping places to the shortest distance at which they
have been reached so far up to this time.
"""
distances = []
dist_so_far = distances[place]
if place not in distances:
distances[place] = dist_so_far
for i in city:
else:
if distances[place]<=dist_so_far:
distances[place] = dist_so_far
#FIXME
#   Consider cases:
#      - Weve never been at place before so its not in distances
#      - Weve been at place before, on a path as short as this one in distances
#      - Weve been here before, but this way is shorter dist_so_far
#    Consider which are base cases, and which require recursion.
#    For the cases that require recursion, what is the progress step?

def main:
"""
Main program gets city pair and map file name,
reports distance or reports lack of connectivity.
"""
parser = argparse.ArgumentParser
description="Find shortest route in road network"
help = "Starting place quoted if it contains blanks"
help = "Destination place quoted if it contains blanks"
help = "Name of file containing road connections and distances"
args = parser.parse_args
start_place  = args.from_place
destination = args.to_place

print"Start place ", start_place, " is not on the map"
exit1
print"Destination ", destination, " is not on the map"
exit1

distances = { }

if destination in distances :
print"Distance from {} to {} is {}".format
start_place,destination, distances[destination]
else:
print"You cant get from {} to {}".formatstart_place, destination

if __name__ == "__main__":
main``````

``````def dfsplace, dist_so_far, roads, distances:
"""Depth-first search, which may continue from from_place if dist_so_far
is the shortest distance at which it has yet been reached.
Args:
place: Currently searching from here
dist_so_far:  Distance at which from_place has been reached
this time which may not be the shortest path to from_place
roads:  dict mapping places to lists of hops of the form place, hop-distance
distances: dict mapping places to the shortest distance at which they
have been reached so far up to this time.
"""

if place not in distances:
distances[place] = dist_so_far
else:
if distances[place] > dist_so_far:
distances[place] = dist_so_far

``````def dfsplace, dist_so_far,roads,distances:
if place not in distances or place in distances and dist_so_far < distances[place]:
distances[place] = dist_so_far
next_place, dist = routes
dfsnext_place, dist_so_far + dist, roads, distances``````

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