HDU - 1175 "连连看" (DFS+剪枝回溯)

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1175

DFS的要点就在于剪枝。
重点是判断：转弯2次时，下一步的方向是否为目标所在方向。

代码如下：

#include<cstdio>
using namespace std;
const int maxn = 1000 + 5;
const int dir[4][2] = { {-1,0},{0,-1},{1,0},{0,1} };
int n, m, t, a[maxn][maxn];
int row, col, row2, col2;
bool ok;
bool check(int row, int col) {
	if (row < 1 || col<1 || row>n || col>m)return false;
	return true;
}
inline void dfs(int row, int col, int before, int sum) {
	if (ok)return;
	for (int i = 0; i < 4; i++) {
		if (sum == 2) {
			if (i != before)continue;
			if (row == row2) {
				if (col2 > col) {
					if (i != 3)continue;
				}
				else if (i != 1)continue;
			}
			else if (col == col2) {
				if (row2 > row) {
					if (i != 2)continue;
				}
				else if (i != 0)continue;
			}
			else return;
		}
		int nextr = row + dir[i][0];
		int nextc = col + dir[i][1];
		if (nextr == row2 && nextc == col2) {
			ok = true; return;
		}
		if (check(nextr, nextc) && !a[nextr][nextc]) {
			if (before != i) dfs(nextr, nextc, i, sum + 1);
			else dfs(nextr, nextc, i, sum);
		}
	}
}

int main(void) {
	while (~scanf("%d %d", &n, &m) && n) {
		for (int i = 1; i <= n; i++) {
			for (int j = 1; j <= m; j++) {
				scanf("%d", &a[i][j]);
			}
		}
		scanf("%d", &t);
		while (t--) {
			ok = false;
			scanf("%d %d %d %d", &row, &col, &row2, &col2);
			if (!a[row][col] || !a[row2][col2] || a[row][col] != a[row2][col2]) {
				printf("NO\n"); continue;
			}
			for (int i = 0; i < 4; i++) 
				dfs(row, col, i, 0);
			if (ok)printf("YES\n");
			else printf("NO\n");
		}
	}
	return 0;
}