|
|
package class_2022_03_4_week;
|
|
|
|
|
|
// 来自理想汽车
|
|
|
// a -> b，代表a在食物链中被b捕食
|
|
|
// 给定一个有向无环图，返回
|
|
|
// 这个图中从最初级动物到最顶级捕食者的食物链有几条
|
|
|
// 线上测试链接 : https://www.luogu.com.cn/problem/P4017
|
|
|
// 以下代码都提交，提交时把主类名改成"Main"即可
|
|
|
// 注意：洛谷测试平台对java提交非常不友好，空间限制可能会卡住，C++的提交就完全不会
|
|
|
// 所以提交时如果显示失败，就多提交几次，一定是能成功的
|
|
|
// 这道题本身就是用到拓扑排序，没什么特别的
|
|
|
// 但是为了提交能通过，逼迫我在空间上做的优化值得好好说一下，可以推广到其他题目
|
|
|
import java.util.Arrays;
|
|
|
import java.util.Scanner;
|
|
|
|
|
|
public class Code05_HowManyWaysFromBottomToTop {
|
|
|
|
|
|
public static int[] in = new int[5001];
|
|
|
public static boolean[] out = new boolean[5001];
|
|
|
public static int[] lines = new int[5001];
|
|
|
public static int[] headEdge = new int[5001];
|
|
|
public static int[] queue = new int[5001];
|
|
|
public static int mod = 80112002;
|
|
|
public static int n = 0;
|
|
|
|
|
|
public static void main(String[] args) {
|
|
|
Scanner sc = new Scanner(System.in);
|
|
|
while (sc.hasNext()) {
|
|
|
Arrays.fill(in, 0);
|
|
|
Arrays.fill(out, false);
|
|
|
Arrays.fill(lines, 0);
|
|
|
Arrays.fill(headEdge, 0);
|
|
|
n = sc.nextInt();
|
|
|
int m = sc.nextInt();
|
|
|
int[] preEdge = new int[m + 1];
|
|
|
int[] edgesTo = new int[m + 1];
|
|
|
for (int i = 1; i <= m; i++) {
|
|
|
int from = sc.nextInt();
|
|
|
int to = sc.nextInt();
|
|
|
edgesTo[i] = to;
|
|
|
preEdge[i] = headEdge[from];
|
|
|
headEdge[from] = i;
|
|
|
out[from] = true;
|
|
|
in[to]++;
|
|
|
}
|
|
|
System.out.println(howManyWays(preEdge, edgesTo));
|
|
|
}
|
|
|
sc.close();
|
|
|
}
|
|
|
|
|
|
public static int howManyWays(int[] preEdge, int[] edgesTo) {
|
|
|
int ql = 0;
|
|
|
int qr = 0;
|
|
|
for (int i = 1; i <= n; i++) {
|
|
|
if (in[i] == 0) {
|
|
|
queue[qr++] = i;
|
|
|
lines[i] = 1;
|
|
|
}
|
|
|
}
|
|
|
while (ql < qr) {
|
|
|
int cur = queue[ql++];
|
|
|
int edge = headEdge[cur];
|
|
|
while (edge != 0) {
|
|
|
int next = edgesTo[edge];
|
|
|
lines[next] = (lines[next] + lines[cur]) % mod;
|
|
|
if (--in[next] == 0) {
|
|
|
queue[qr++] = next;
|
|
|
}
|
|
|
edge = preEdge[edge];
|
|
|
}
|
|
|
}
|
|
|
int ans = 0;
|
|
|
for (int i = 1; i <= n; i++) {
|
|
|
if (!out[i]) {
|
|
|
ans = (ans + lines[i]) % mod;
|
|
|
}
|
|
|
}
|
|
|
return ans;
|
|
|
}
|
|
|
|
|
|
}
|
