[topcoder]NinePuzzle

http://community.topcoder.com/stat?c=problem_statement&pm=11225&rd=14427

http://apps.topcoder.com/wiki/display/tc/SRM+498

此题的暴力解法是一个BFS。BFS的话，要有Queue，如果是图的话，还要有Set来记录走过的状态。后面答案中会看到其实可以用贪心，就是只要输入和输出的各种颜色数量一样，就一定能变换过来，这个下回再表。

import java.util.*;
public class NinePuzzle
{
    public int getMinimumCost(String init, String goal)
    {
        int[][] arr = new int[10][];
        arr[0] = new int[]{1, 2};
        arr[1] = new int[]{0, 2, 3, 4};
        arr[2] = new int[]{0, 1, 4, 5};
        arr[3] = new int[]{1, 4, 6, 7};
        arr[4] = new int[]{1, 2, 3, 5, 7, 8};
        arr[5] = new int[]{2, 4, 8, 9};
        arr[6] = new int[]{3, 7};
        arr[7] = new int[]{3, 4, 6, 8};
        arr[8] = new int[]{4, 5, 7, 9};
        arr[9] = new int[]{5, 8};
        Queue<String> queue = new LinkedList<String>();
        HashSet<String> set = new HashSet<String>();
        set.add(goal);
        queue.offer(goal);
        int initStar = -1;
        for (int i = 0; i < init.length(); i++)
            if (init.charAt(i) == '*')
                initStar = i;
        int ans = Integer.MAX_VALUE;
        while (queue.size() != 0)
        {
            String s = queue.poll();
            char[] ca = s.toCharArray();
            int starPos = -1;
            for (int i = 0; i < ca.length; i++)
                if (ca[i] == '*') starPos = i;

            if (initStar == starPos)
            {
                int a = 0;
                for (int i = 0; i < s.length(); i++)
                {
                    if (initStar != i && ca[i] != init.charAt(i)) a++;
                }
                if (ans > a) ans = a;
            }
            for (int i = 0; i < arr[starPos].length; i++)
            {
                swap(ca, starPos, arr[starPos][i]);
                String tmp = new String(ca);
                if (!set.contains(tmp))
                {
                    queue.offer(tmp);
                    set.add(tmp);
                }
                swap(ca, starPos, arr[starPos][i]);
            }
        }
        return ans;
    }

    private void swap(char[] ca, int i, int j)
    {
        char tmp = ca[i];
        ca[i] = ca[j];
        ca[j] = tmp;
    }
}