DWITE Online Computer Programming Contest

Up To Four Colours

December 2009
Problem 5

One of the ways to print T-shirt designs is by a process called screen printing – a design is broken up into “screens” that allow paint to fill certain solid regions. This results in a higher quality print, but requires additional setup (and thus cost) for every colour used.

Having started what we hope to turn out to be a successful T-shirt design company, we want to minimize the printing costs. Each design can be modelled as a graph, and we want to be able to colour it such that no two adjacent nodes are of the same colour (think of it as a map of countries – every country border touches two different colours). What is the minimum number of colours that each design requires?. Let’s assume that paying for more than 4 is too much, so if we can’t colour it in 4, we’ll give up.

The input file DATA5.txt will contain 5 sets of input. For each set, the first line will contain a positive integer 1 ≤ N ≤ 20 representing the number of edges in a graph, followed by N lines describing the graph. Each such line will contain two positive integers separated by a single space – a connection between two nodes identified by such integers. If both node IDs are the same, treat it as a disconnected node (there is no edge to itself).

The output file OUT5.txt will contain 5 lines, each of which is the minimum number of colours required to colour the graph, or “0” if four is not enough.

Sample Input:
```2
1 1
2 2
4
1 2
2 3
3 4
4 1
5
1 2
2 3
3 4
4 5
5 1
10
1 2
2 3
3 4
4 5
5 1
1 3
1 4
2 4
2 5
3 5
10
1 2
2 3
3 4
4 5
5 1
6 1
6 2
6 3
6 4
6 5```
Sample Output:
```1
2
3
0
4```