D. Unbearable Controversy of Being

 

 http://codeforces.com/contest/489/problem/D

 

 

暴力，枚举所有"damn rhombus"

 

对于任意a、c，设其a->i->c的路个数为cnt，其"damn rhombus"个数为cnt*（cnt-1）/2

 

初始化cnt[][]，然后遍历所有点即可

 

 

public class Main {
    public static void main(String[] args) {
        Scanner io = new Scanner(System.in);
        int n = io.nextInt(), m = io.nextInt();
        //链表存图，节约时间
        int[][] g = new int[3010][3010];
        int[][] cnt = new int[3010][3010];

        for (int i = 0; i < m; i++) {
            int a = io.nextInt(), b = io.nextInt();
            g[a][++g[a][0]] = b;
        }
        //初始化cnt，避免在两遍n循环中重复计算，节约时间
        for (int i=1;i<=n;i++)for (int j=1;j<=g[i][0];j++){
            int c=g[i][j];
            for (int k=1;k<=g[c][0];k++)cnt[i][g[c][k]]++;
        }

        int ans = 0;
        for (int a = 1; a <= n; a++) {
            for (int c = 1; c <= n; c++) {
                if (c == a) continue;
                ans += cnt[a][c] * (cnt[a][c] - 1) / 2;
            }
        }
        System.out.println(ans);
    }
}