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Problem solution · Java

CCC 2009 S4 - Ship and Shop

CCC 2009 S4 - Ship and Shop: a Java solution using depth-first search. Learn the idea, check the complexity, and read the full code, with credit to CCCSolutions.

Technique
Depth-first search
Source
CCCSolutions
Length
107 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Depth-first search

For CCC 2009 S4 - Ship and Shop, the implementation follows one branch at a time, making it suitable for components, trees, backtracking, or dependency exploration.

  1. Define the state carried into one recursive or stack frame.
  2. Mark or choose the current state before exploring children.
  3. Combine child results or undo the choice when the branch finishes.

Code notes

  • 107 lines of Java from the credited upstream file ccc09s4.java.
  • The implementation visibly relies on sequence storage.
  • 7 loop blocks detected.

Complexity

Count unique states for graph traversal; for backtracking, count the branching factor and maximum depth.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from CCCSolutions by CCCSolutions contributors · Milliken Mills High School and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeCCC 2009 S4 - Ship and Shop · JavaJava
Use this to learn the idea, then write your own version.
// CCC 2009//// S4: Ship and Shop//// straight recursion, tracking if you visited a city before, too// help recursion speed up.//// It works fines for data 1, 2, and 4. // to make it work for 3 and 5, i have to loop thru it again// using the better values of shop, until there are no changes. ////  reading takes ~20 sec for large files//  recursion takes ~5 sec.// import java.awt.*;import hsa.*; public class CCC2009S4ShipAndShop{     public static int[] [] ship;  // lower triangular 2d array    public static int[] shop;    public static boolean[] visited;    public static boolean improvement;    public static int n, d;    public static Console c;     public static void main (String[] args)    { 	c = new Console ();	TextInputFile f = new TextInputFile ("s4.5.in"); 	n = f.readInt ();	ship = new int [n + 1] [];	shop = new int [n + 1];	visited = new boolean [n + 1];	for (int i = 1 ; i <= n ; i++)	{	    ship [i] = new int [i + 1];	    shop [i] = 999999999;	    visited [i] = false;	    for (int j = 1 ; j < i ; j++)		ship [i] [j] = 999999999;	    ship [i] [i] = 0;	} 	int t = f.readInt ();	int i, j, x;	for (int k = 0 ; k < t ; k++)	{	    i = f.readInt ();	    j = f.readInt ();	    x = f.readInt ();	    if (i < j && j <= n)		ship [j] [i] = x;	    else if (j < i && i <= n)		ship [i] [j] = x;	}	int k = f.readInt ();	for (i = 0 ; i < k ; i++)	{	    j = f.readInt ();	    x = f.readInt ();	    if (j <= n)		shop [j] = x;	}	d = f.readInt ();	c.println ("Input finished.");		// this is to repeat the recursion, until the answer 	// remains unchanged. I'd never have thought to do this	// unless I had the real data to work with. so I suppose this 	// shows the error of my method..... it works though :-)	int old = 0;	x = cheapest(d);	while (x != old)	{	   old  = x;	   for (i = 1 ; i <= n ; i++)		visited [i] = false;	   x = cheapest (d);	}	c.println (x);    }      // the cheapest city x can get the pencil is    // the cheapest it can get it from any of the other cities, including    // shipping. (Answers are stored in shop array.)    public static int cheapest (int x)    {	if (visited [x])	    return shop [x];	visited [x] = true;	int smallest = shop [x];	for (int i = 1 ; i <= n ; i++)	    if (ship [Math.max (i, x)] [Math.min (i, x)] + cheapest (i) < smallest)		smallest = ship [Math.max (i, x)] [Math.min (i, x)] + cheapest (i);	shop [x] = smallest;	return smallest;    }}

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