diff --git a/src/lib.rs b/src/lib.rs
index 5786df9..3cacac6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,19 +1,6 @@
 pub mod p1;
 pub mod p2;
 pub mod p238;
+pub mod p3;
 
 pub struct Solution;
-
-// Definition for singly-linked list.
-#[derive(PartialEq, Eq, Clone, Debug)]
-pub struct ListNode {
-    pub val: i32,
-    pub next: Option<Box<ListNode>>,
-}
-
-impl ListNode {
-    #[inline]
-    fn _new(val: i32) -> Self {
-        ListNode { next: None, val }
-    }
-}
diff --git a/src/p2.rs b/src/p2.rs
index 846f311..ab4ea54 100644
--- a/src/p2.rs
+++ b/src/p2.rs
@@ -1,6 +1,11 @@
-use crate::{ListNode, Solution};
+use crate::Solution;
 
 // Definition for singly-linked list.
+#[derive(PartialEq, Eq, Clone, Debug)]
+pub struct ListNode {
+    pub val: i32,
+    pub next: Option<Box<ListNode>>,
+}
 
 impl Solution {
     pub fn add_two_numbers(
diff --git a/src/p3.rs b/src/p3.rs
new file mode 100644
index 0000000..1cd4d03
--- /dev/null
+++ b/src/p3.rs
@@ -0,0 +1,74 @@
+use crate::Solution;
+
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn length_of_longest_substring(s: String) -> i32 {
+        if s.is_empty() {
+            return 0;
+        }
+        let mut letters: HashMap<char, usize> = HashMap::new();
+        let mut cur_start: usize = 0;
+        let mut max_length = 0;
+        for (i, c) in s.chars().enumerate() {
+            if max_length > i - cur_start && max_length >= s.len() - cur_start {
+                return max_length as i32 + 1;
+            }
+            if let Some(idx) = letters.get(&c)
+                && *idx >= cur_start
+            {
+                cur_start = idx + 1;
+                *letters.get_mut(&c).unwrap() = i;
+            } else {
+                letters.insert(c, i);
+                if (i - cur_start) > max_length {
+                    max_length = i - cur_start;
+                }
+            }
+        }
+        max_length as i32 + 1
+    }
+}
+
+// println!("{cur_start} {i} {max_length} {c} {letters:?}");
+#[test]
+pub fn test_length_of_longest_substring_1() {
+    assert_eq!(
+        Solution::length_of_longest_substring("abcabcbb".to_string()),
+        3
+    );
+}
+
+#[test]
+pub fn test_length_of_longest_substring_2() {
+    assert_eq!(
+        Solution::length_of_longest_substring("bbbbb".to_string()),
+        1
+    );
+}
+
+#[test]
+pub fn test_length_of_longest_substring_3() {
+    assert_eq!(
+        Solution::length_of_longest_substring("pwwkew".to_string()),
+        3
+    );
+}
+
+#[test]
+pub fn test_length_of_longest_substring_4() {
+    assert_eq!(Solution::length_of_longest_substring("au".to_string()), 2);
+}
+
+#[test]
+pub fn test_length_of_longest_substring_5() {
+    assert_eq!(Solution::length_of_longest_substring("dvdf".to_string()), 3);
+}
+
+#[test]
+pub fn test_length_of_longest_substring_6() {
+    assert_eq!(
+        Solution::length_of_longest_substring("tmmzuxt".to_string()),
+        5
+    );
+}