[ABC262B] Triangle (Easier)

Problem Statement

You are given a simple undirected graph with $N$ vertices and $M$ edges. The vertices are numbered $1, \dots, N$, and the $i$-th $(1 \leq i \leq M)$ edge connects Vertex $U_i$ and Vertex $V_i$.

Find the number of tuples of integers $a, b, c$ that satisfy all of the following conditions:

• $1 \leq a \lt b \lt c \leq N$
• There is an edge connecting Vertex $a$ and Vertex $b$.
• There is an edge connecting Vertex $b$ and Vertex $c$.
• There is an edge connecting Vertex $c$ and Vertex $a$.

Constraints

• $3 \leq N \leq 100$
• $1 \leq M \leq \frac{N(N - 1)}{2}$
• $1 \leq U_i \lt V_i \leq N \, (1 \leq i \leq M)$
• $(U_i, V_i) \neq (U_j, V_j) \, (i \neq j)$
• All values in input are integers.

---

Input

Input is given from Standard Input in the following format:

$N$ $M$
$U_1$ $V_1$
$\vdots$
$U_M$ $V_M$

Output

Print the answer.

---

Sample Input 1

5 6
1 5
4 5
2 3
1 4
3 5
2 5

Sample Output 1

2

$(a, b, c) = (1, 4, 5), (2, 3, 5)$ satisfy the conditions.

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Sample Input 2

3 1
1 2

Sample Output 2

0

---

Sample Input 3

7 10
1 7
5 7
2 5
3 6
4 7
1 5
2 4
1 3
1 6
2 7

用邻接矩阵存边，暴力枚举三个数，用邻接矩阵判断他们是否成三元环。

#include<cstdio>
const int N=105;
int n,m,u,v,e[N][N],cnt;
int main()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++)
	{
		scanf("%d%d",&u,&v);
		e[u][v]=e[v][u]=1;
	}
	for(int i=1;i<n;i++)
		for(int j=i+1;j<n;j++)
			for(int k=j+1;k<=n;k++)
				if(e[i][j]&&e[j][k]&&e[i][k])
					++cnt;
	printf("%d",cnt); 
}