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  1.  
  2. import java.io.BufferedInputStream;
  3. import java.io.IOException;
  4.  
  5. public class Main {
  6.     static class FastScanner {
  7.         private final BufferedInputStream in = new BufferedInputStream(System.in);
  8.  
  9.         int nextInt() throws IOException {
  10.             int c;
  11.             do {
  12.                 c = in.read();
  13.             } while (c <= ' ' && c != -1);
  14.  
  15.             int num = 0;
  16.             while (c > ' ') {
  17.                 num = num * 10 + c - '0';
  18.                 c = in.read();
  19.             }
  20.             return num;
  21.         }
  22.     }
  23.  
  24.     public static void main(String[] args) throws IOException {
  25.         FastScanner fs = new FastScanner();
  26.  
  27.         int n = fs.nextInt();
  28.         int m = fs.nextInt();
  29.  
  30.         int[] head = new int[n + 1];
  31.         int[] to = new int[2 * m];
  32.         int[] next = new int[2 * m];
  33.         int[] degree = new int[n + 1];
  34.  
  35.         java.util.Arrays.fill(head, -1);
  36.  
  37.         int edgeIndex = 0;
  38.  
  39.         for (int i = 0; i < m; i++) {
  40.             int u = fs.nextInt();
  41.             int v = fs.nextInt();
  42.  
  43.             to[edgeIndex] = v;
  44.             next[edgeIndex] = head[u];
  45.             head[u] = edgeIndex++;
  46.  
  47.             to[edgeIndex] = u;
  48.             next[edgeIndex] = head[v];
  49.             head[v] = edgeIndex++;
  50.  
  51.             degree[u]++;
  52.             degree[v]++;
  53.         }
  54.  
  55.         boolean[] visited = new boolean[n + 1];
  56.         int[] stack = new int[n];
  57.         int answer = 0;
  58.  
  59.         for (int i = 1; i <= n; i++) {
  60.             if (visited[i]) {
  61.                 continue;
  62.             }
  63.  
  64.             int top = 0;
  65.             stack[top++] = i;
  66.             visited[i] = true;
  67.  
  68.             boolean isCycle = true;
  69.  
  70.             while (top > 0) {
  71.                 int u = stack[--top];
  72.  
  73.                 if (degree[u] != 2) {
  74.                     isCycle = false;
  75.                 }
  76.  
  77.                 for (int e = head[u]; e != -1; e = next[e]) {
  78.                     int v = to[e];
  79.  
  80.                     if (!visited[v]) {
  81.                         visited[v] = true;
  82.                         stack[top++] = v;
  83.                     }
  84.                 }
  85.             }
  86.  
  87.             if (isCycle) {
  88.                 answer++;
  89.             }
  90.         }
  91.  
  92.         System.out.println(answer);
  93.     }
  94. }
  95.  
Success #stdin #stdout 0.07s 40376KB
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https://ideone.com/X586of
language:
Java (12.0.1)
created:
5 hours ago
visibility:
public

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