ADD file via upload

src/PrimaryProblem.java

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+import java.util.Random;
+
+public class PrimaryProblem implements ProblemsGenerator {
+  private static final Random RANDOM = new Random();
+  private static final char[] OPS = {'+', '-', '*', '/'};
+  
+  @Override
+  public String generate() {
+    int count = RANDOM.nextInt(5) + 1; // 1~5个数
+    StringBuilder sb = new StringBuilder();
+    for (int i = 0; i < count; i++) {
+      int num = RANDOM.nextInt(100) + 1;
+      sb.append(num);
+      if (i < count - 1) {
+        sb.append(" ").append(OPS[RANDOM.nextInt(OPS.length)]).append(" ");
+      }
+    }
+    return sb.toString();
+  }
+}