Update PrimaryProblem.java

src/PrimaryProblem.java

@@ -2,17 +2,41 @@ import java.util.Random;
 
 public class PrimaryProblem implements ProblemsGenerator {
   private static final Random RANDOM = new Random();
-  private static final char[] OPS = {'+', '-', '*', '/'};
+  private static final String[] OPERATORS = {"+", "-", "*", "/"};
   
   @Override
   public String generate() {
-    int count = RANDOM.nextInt(5) + 1; // 1~5个数
+    int count = RANDOM.nextInt(4) + 2;
     StringBuilder sb = new StringBuilder();
+    
+    boolean addParentheses = count >= 3 && RANDOM.nextDouble() < 0.5;
+    if (addParentheses) {
+      int parenCount = RANDOM.nextInt(count - 2) + 2;
+      int startPos = RANDOM.nextInt(count - parenCount + 1);
+      
+      for (int i = 0; i < count; i++) {
+        if (i == startPos) {
+          sb.append("( ");
+        }
+        
+        int num = RANDOM.nextInt(100) + 1;
+        sb.append(num);
+        
+        if (i == startPos + parenCount - 1) {
+          sb.append(" )");
+        }
+        
+        if (i < count - 1) {
+          sb.append(" ").append(OPERATORS[RANDOM.nextInt(OPERATORS.length)]).append(" ");
+        }
+      }
+    } else {
       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(" ");
+          sb.append(" ").append(OPERATORS[RANDOM.nextInt(OPERATORS.length)]).append(" ");
+        }
       }
     }
     return sb.toString();