Update PrimaryProblem.java

src/PrimaryProblem.java

@@ -1,20 +1,44 @@
-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();
-  }
+import java.util.Random;
+
+public class PrimaryProblem implements ProblemsGenerator {
+  private static final Random RANDOM = new Random();
+  private static final String[] OPERATORS = {"+", "-", "*", "/"};
+  
+  @Override
+  public String generate() {
+    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(OPERATORS[RANDOM.nextInt(OPERATORS.length)]).append(" ");
+        }
+      }
+    }
+    return sb.toString();
+  }
 }