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diff --git a/‎src/main/java/com/hezhou/algorithm/chapter1/section1/Ex31.java‎
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+package com.hezhou.algorithm.chapter1.section1;
+
+import edu.princeton.cs.algs4.StdDraw;
+import edu.princeton.cs.algs4.StdRandom;
+
+public class Ex31 {
+  static class Point {
+    double x;
+    double y;
+
+    public Point(double x, double y) {
+      this.x = x;
+      this.y = y;
+    }
+  }
+
+  public static void main(String[] args) {
+    int N = Integer.parseInt(args[0]);
+    double p = Double.parseDouble(args[1]);
+
+    double radius = 0.5;
+    StdDraw.circle(0.5, 0.5, radius);
+    StdDraw.setPenRadius(0.05);
+    StdDraw.setPenColor(StdDraw.RED);
+
+    double average = Math.PI * 2 / N;
+    Point[] points = new Point[N];
+    for (int i = 0; i < N; i++) {
+      double angle = average * i;
+      double x = radius + radius * Math.cos(angle);
+      double y = radius + radius * Math.sin(angle);
+      points[i] = new Point(x, y);
+      StdDraw.point(x, y);
+    }
+
+    StdDraw.setPenRadius(0.01);
+    StdDraw.setPenColor(StdDraw.GRAY);
+    for (int i = 0; i < N; i++) {
+      for (int j = i + 1; j < N; j++) {
+        if (StdRandom.bernoulli(p)) {
+          StdDraw.line(points[i].x, points[i].y, points[j].x, points[j].y);
+        }
+      }
+    }
+  }
+}