diff --git a/src/main/java/com/cool/ling_cha_mount/sliding_windows/Num3090.java b/src/main/java/com/cool/ling_cha_mount/sliding_windows/Num3090.java
new file mode 100644
index 0000000..7483689
--- /dev/null
+++ b/src/main/java/com/cool/ling_cha_mount/sliding_windows/Num3090.java
@@ -0,0 +1,31 @@
+package com.cool.ling_cha_mount.sliding_windows;
+
+/**
+ * Created with IntelliJ IDEA.
+ *
+ * @Author: Cool
+ * @Date: 2024/10/16/17:05
+ * @Description: 3090. 每个字符最多出现两次的最长子字符串
+ * Level 2
+ * Hard 1
+ * Score 1329
+ */
+public class Num3090 {
+
+    public int maximumLengthSubstring(String s) {
+        int res=0;
+        int[] arr=new int[26];
+        int left=0;
+        for(int i=0;i<s.length();i++){
+            if(++arr[s.charAt(i)-'a']>2){
+                while(left<i){
+                    if(--arr[s.charAt(left++)-'a']==2){
+                        break;
+                    }
+                }
+            }
+            res=Math.max(res,i-left+1);
+        }
+        return res;
+    }
+}