application3-2.cpp

对于特定长度的高速公路

// Search for a route. 
#include <iostream> 
#include <stack> 
#include <string> 
#include <vector> 
 
using namespace std; 
  
// Flight information.  
struct FlightInfo {  
  string from;  // departure city 
  string to;    // destination city 
  int distance; // distance between from and to 
  bool skip;    // used in backtracking  
  
  FlightInfo() { 
    from = ""; 
    to = ""; 
    distance = 0; 
    skip = false; 
  } 
 
  FlightInfo(string f, string t, int d) {  
    from = f;  
    to = t;  
    distance = d;  
    skip = false;  
  }  
};  
 
// Find connections using a depth-first search.  
class Search {  
  // This vector holds the flight information.  
  vector<FlightInfo> flights; 
 
  // This statck is used for backtracking.  
  stack<FlightInfo> btStack;  
 
  // If there is a flight between from and to,  
  // store the distance of the flight in dist. 
  // Return true if the flight exists and, 
  // false otherwise. 
  bool match(string from, string to, int &dist); 
 
  // Given from, find any connection.  
  // Return true if a connection is found, 
  // and false otherwise. 
  bool find(string from, FlightInfo &f); 
 
public:  
 
  // Put flights into the database.  
  void addflight(string from, string to, int dist) {  
    flights.push_back(FlightInfo(from, to, dist));  
  }  
 //Reset the skip fields in flights vector.
  void resetSkip(FlightInfo f);
  // Show the route and total distance.  
  void route();  
 
  // Determine if there is a route between from and to.   
  void findroute(string from, string to); 
 
  // Return true if a route has been found. 
  bool routefound() { 
    return !btStack.empty(); 
  } 
};  

// Show the route and total distance.  
void Search::route()  
{  
  stack<FlightInfo> rev;  
  int dist = 0;  
  FlightInfo f;  
  
  // Reverse the stack to display route.  
  while(!btStack.empty()) { 
    f = btStack.top(); 
    rev.push(f); 
    btStack.pop(); 
  } 
  
  // Display the route. 
  while(!rev.empty()) { 
    f = rev.top(); 
    rev.pop();  
    cout << f.from << " to ";  
    dist += f.distance;  
  }  
  
  cout << f.to << endl;  
  cout << "Distance is " << dist << endl;  
}  
  
// If there is a flight between from and to,  
// store the distance of the flight in dist. 
// Return true if the flight exists and, 
// false otherwise. 
bool Search::match(string from, string to, int &dist)  
{  
  for(unsigned i=0; i < flights.size(); i++) {  
    if(flights[i].from == from &&  
       flights[i].to == to && !flights[i].skip)  
    {  
      flights[i].skip = true; // prevent reuse  
      dist = flights[i].distance; 
      return true; 
    }  
  }  
  
  return false; // not found   
}  
    
// Given from, find any connection.  
// Return true if a connection is found, 
// and false otherwise. 
bool Search::find(string from, FlightInfo &f)  
{  
  for(unsigned i=0; i < flights.size(); i++) {  
    if(flights[i].from == from && !flights[i].skip) {  
      f = flights[i]; 
      flights[i].skip = true; // prevent reuse  
  
      return true;  
    }  
  }  
  
  return false;  
}  
  
// Breadth-first version. 
// Determine if there is a route between from and to. 
void Search::findroute(string from, string to)  
{  
  int dist;  
  FlightInfo f;  
  
  // This stack is needed by the breadth-first search.  
  stack<FlightInfo> resetStck; 
  
  // See if at destination.  
  if(match(from, to, dist)) {  
    btStack.push(FlightInfo(from, to, dist));  
    return;  
  }  
  
  // Following is the first part of the breadth-first  
  // modification.  It checks all connecting flights  
  // from a specified node. 
  while(find(from, f)) {  
    resetStck.push(f);  
    if(match(f.to, to, dist)) {  
      resetStck.push(FlightInfo(f));  
      btStack.push(FlightInfo(from, f.to, f.distance));  
      btStack.push(FlightInfo(f.to, to, dist));  
      return;  
    }  
  }  
  
  // The following code resets the skip fields set by  
  // preceding while loop. This is also part of the  
  // breadth-first modifiction. 
  while(!resetStck.empty()) { 
    resetSkip(resetStck.top()); 
    resetStck.pop(); 
  }  
  
  // Try another connection.  
  if(find(from, f)) {  
    btStack.push(FlightInfo(from, to, f.distance));  
    findroute(f.to, to);  
  }  
  else if(!btStack.empty()) {  
    // Backtrack and try another connection.  
    f = btStack.top(); 
    btStack.pop();  
    findroute(f.from, f.to);  
  }  
}

 //Reset the skip fields in flights vector.
void Search::resetSkip(FlightInfo f)
{
	for (unsigned i=0; i<flights.size(); i++)
		if (flights[i].from ==f.from && flights[i].to ==f.to)
			flights[i].skip = false;
}

int main() {  
  char to[40], from[40]; 
  Search ob; 
 
  // Add flight connections to database. 
  ob.addflight("New York", "Chicago", 900);  
  ob.addflight("Chicago", "Denver", 1000);  
  ob.addflight("New York", "Toronto", 500);  
  ob.addflight("New York", "Denver", 1800);  
  ob.addflight("Toronto", "Calgary", 1700);  
  ob.addflight("Toronto", "Los Angeles", 2500);  
  ob.addflight("Toronto", "Chicago", 500);  
  ob.addflight("Denver", "Urbana", 1000);  
  ob.addflight("Denver", "Houston", 1000);  
  ob.addflight("Houston", "Los Angeles", 1500);  
  ob.addflight("Denver", "Los Angeles", 1000);  
 
  // Get departure and destination cities.  
  cout << "From? ";  
 
  cin.getline(from, 40); 
  cout << "To? ";  
 
  cin.getline(to, 40); 
 
  // See if there is a route between from and to. 
  ob.findroute(from, to);  
  
  // If there is a route, show it. 
  if(ob.routefound())  
      ob.route();  
 
  return 0; 
}