poj1753

dfs,搜索所有的操作方法，O(2^16)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
using namespace std;

bool map[4][4];
int ans = 100;

void init()
{
	int i, j;

	for (i = 0; i < 4; i++)
	{
		for (j = 0; j < 4; j++)
		{
			char ch;
			cin >> ch;
			if (ch == 'b')
				map[i][j] = true;
			else
				map[i][j] = false;
		}
		getchar();
	}
}

void operate(int x, int y)
{
	if (x < 0 || y < 0 || x > 3 || y > 3)
		return;
	map[x][y] = !map[x][y];
}

void turn(int pos)
{
	int x = pos / 4;
	int y = pos % 4;
	operate(x, y);
	operate(x + 1, y);
	operate(x, y + 1);
	operate(x - 1, y);
	operate(x, y - 1);
}

bool finished()
{
	int tot = 0;
	int i;
	for (i = 0; i < 16; i++)
		tot += map[i / 4][i % 4];
	return ((tot % 16) == 0);
}

void dfs(int pos, int step)
{
	if (finished())
	{
		if (ans > step)
			ans = step;
		return;
	}
	if (pos >= 16)
		return;
	dfs(pos + 1, step);
	turn(pos);
	dfs(pos + 1, step + 1);
	turn(pos);//这句忘了写，导致错误
}

int main()
{
	//freopen("D:\\t.txt", "r", stdin);
	init();
	dfs(0, 0);
	if (ans == 100)
		cout << "Impossible" << endl;
	else
		cout << ans << endl;
	return 0;
}