diff --git a/src/lib.rs b/src/lib.rs
index 7f7b725..00b3c4f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -19,3 +19,6 @@ pub mod p16_3sum_closest;
 pub mod p17_letter_combinations_of_a_phone_number;
 pub mod p18_4sum;
 pub mod p19_remove_nth_node_from_end_of_list;
+pub mod p20_valid_parentheses;
+pub mod p21_merge_two_sorted_lists;
+pub mod p22_generate_parentheses;
diff --git a/src/p12_integer_to_roman.rs b/src/p12_integer_to_roman.rs
index 2ef0a4f..7db55c3 100644
--- a/src/p12_integer_to_roman.rs
+++ b/src/p12_integer_to_roman.rs
@@ -35,15 +35,15 @@ impl Solution {
 
 #[test]
 fn test1() {
-    assert_eq!(Solution::int_to_roman(3), "III".to_string());
+    assert_eq!(Solution::int_to_roman(3), "III");
 }
 
 #[test]
 fn test2() {
-    assert_eq!(Solution::int_to_roman(58), "LVIII".to_string());
+    assert_eq!(Solution::int_to_roman(58), "LVIII");
 }
 
 #[test]
 fn test3() {
-    assert_eq!(Solution::int_to_roman(1994), "MCMXCIV".to_string());
+    assert_eq!(Solution::int_to_roman(1994), "MCMXCIV");
 }
diff --git a/src/p14_longest_common_prefix.rs b/src/p14_longest_common_prefix.rs
index e2f3147..4346b90 100644
--- a/src/p14_longest_common_prefix.rs
+++ b/src/p14_longest_common_prefix.rs
@@ -20,11 +20,11 @@ impl Solution {
 #[test]
 fn test1() {
     let strs = vec!["flower".to_string(),"flow".to_string(),"flight".to_string()];
-    assert_eq!(Solution::longest_common_prefix(strs), "fl".to_string());
+    assert_eq!(Solution::longest_common_prefix(strs), "fl");
 }
 
 #[test]
 fn test2() {
     let strs = vec!["dog".to_string(),"racecar".to_string(),"car".to_string()];
-    assert_eq!(Solution::longest_common_prefix(strs), "".to_string());
+    assert_eq!(Solution::longest_common_prefix(strs), "");
 }
diff --git a/src/p17_letter_combinations_of_a_phone_number.rs b/src/p17_letter_combinations_of_a_phone_number.rs
index 0caf26b..6e99723 100644
--- a/src/p17_letter_combinations_of_a_phone_number.rs
+++ b/src/p17_letter_combinations_of_a_phone_number.rs
@@ -27,11 +27,7 @@ impl Solution {
 fn test1() {
     let mut actual = Solution::letter_combinations("23".to_string());
     actual.sort();
-    assert_eq!(actual, vec![
-        "ad".to_string(), "ae".to_string(), "af".to_string(),
-        "bd".to_string(), "be".to_string(), "bf".to_string(),
-        "cd".to_string(), "ce".to_string(), "cf".to_string()
-    ]);
+    assert_eq!(actual, vec!["ad","ae","af","bd","be","bf","cd","ce","cf"]);
 }
 
 #[test]
@@ -41,6 +37,5 @@ fn test2() {
 
 #[test]
 fn test3() {
-    assert_eq!(Solution::letter_combinations("2".to_string()),
-        vec!["a".to_string(), "b".to_string(), "c".to_string()]);
+    assert_eq!(Solution::letter_combinations("2".to_string()), vec!["a","b","c"]);
 }
diff --git a/src/p20_valid_parentheses.rs b/src/p20_valid_parentheses.rs
new file mode 100644
index 0000000..228265c
--- /dev/null
+++ b/src/p20_valid_parentheses.rs
@@ -0,0 +1,36 @@
+pub struct Solution;
+impl Solution {
+    pub fn is_valid(s: String) -> bool {
+        let s = s.as_bytes();
+        let mut braces = vec![];
+        for i in 0..s.len() {
+            match s[i] {
+                b'(' => braces.push(b')'),
+                b'[' => braces.push(b']'),
+                b'{' => braces.push(b'}'),
+                b')' | b']' | b'}' => {
+                    let Some(brace) = braces.pop()
+                    else { return false };
+                    if brace != s[i] { return false; }
+                },
+                _ => ()
+            }
+        }
+        return braces.is_empty();
+    }
+}
+
+#[test]
+fn test1() {
+    assert_eq!(Solution::is_valid("()".to_string()), true);
+}
+
+#[test]
+fn test2() {
+    assert_eq!(Solution::is_valid("()[]{}".to_string()), true);
+}
+
+#[test]
+fn test3() {
+    assert_eq!(Solution::is_valid("(]".to_string()), false);
+}
diff --git a/src/p21_merge_two_sorted_lists.rs b/src/p21_merge_two_sorted_lists.rs
new file mode 100644
index 0000000..55cd7cf
--- /dev/null
+++ b/src/p21_merge_two_sorted_lists.rs
@@ -0,0 +1,43 @@
+use super::linked_list::ListNode;
+
+pub struct Solution;
+impl Solution {
+    pub fn merge_two_lists(list1: Option<Box<ListNode>>, list2: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
+        let mut head = None;
+        let mut tail = &mut head;
+        let (mut l1, mut l2) = (&list1, &list2);
+        while l1.is_some() || l2.is_some() {
+            let val1 = if l1.is_some() {
+                l1.as_ref().unwrap().val
+            } else { i32::MAX };
+            let val2 = if l2.is_some() {
+                l2.as_ref().unwrap().val
+            } else { i32::MAX };
+            let num = if val1 < val2 {
+                l1 = &l1.as_ref().unwrap().next;
+                val1
+            } else {
+                l2 = &l2.as_ref().unwrap().next;
+                val2
+            };
+            *tail = Some(Box::new(ListNode::new(num)));
+            tail = &mut tail.as_mut().unwrap().next;
+        }
+        return head;
+    }
+}
+
+#[test]
+fn test1() {
+    assert_eq!(Solution::merge_two_lists(list![1,2,4], list![1,3,4]), list![1,1,2,3,4,4]);
+}
+
+#[test]
+fn test2() {
+    assert_eq!(Solution::merge_two_lists(list![], list![]), list![]);
+}
+
+#[test]
+fn test3() {
+    assert_eq!(Solution::merge_two_lists(list![], list![0]), list![0]);
+}
diff --git a/src/p22_generate_parentheses.rs b/src/p22_generate_parentheses.rs
new file mode 100644
index 0000000..1f44d60
--- /dev/null
+++ b/src/p22_generate_parentheses.rs
@@ -0,0 +1,14 @@
+pub struct Solution;
+impl Solution {
+    pub fn generate_parenthesis(n: i32) -> Vec<String> {
+        
+        return vec![];
+    }
+}
+
+#[test]
+fn test1() {
+    let mut actual = Solution::generate_parenthesis(3);
+    actual.sort();
+    assert_eq!(actual, vec!["((()))","(()())","(())()","()(())","()()()"]);
+}
diff --git a/src/p5_longest_palindrome_substring.rs b/src/p5_longest_palindrome_substring.rs
index 04a7342..e170242 100644
--- a/src/p5_longest_palindrome_substring.rs
+++ b/src/p5_longest_palindrome_substring.rs
@@ -18,10 +18,10 @@ impl Solution {
 #[test]
 fn test1() {
     let actual = Solution::longest_palindrome("babad".to_string());
-    assert!(actual == "bab".to_string() || actual == "aba".to_string());
+    assert!(actual == "bab" || actual == "aba");
 }
 
 #[test]
 fn test2() {
-    assert_eq!(Solution::longest_palindrome("cbbd".to_string()), "bb".to_string());
+    assert_eq!(Solution::longest_palindrome("cbbd".to_string()), "bb");
 }
diff --git a/src/p6_zigzag_conversion.rs b/src/p6_zigzag_conversion.rs
index 1946c6e..f45bbbf 100644
--- a/src/p6_zigzag_conversion.rs
+++ b/src/p6_zigzag_conversion.rs
@@ -20,15 +20,15 @@ impl Solution {
 
 #[test]
 fn test1() {
-    assert_eq!(Solution::convert("PAYPALISHIRING".to_string(), 3), "PAHNAPLSIIGYIR".to_string());
+    assert_eq!(Solution::convert("PAYPALISHIRING".to_string(), 3), "PAHNAPLSIIGYIR");
 }
 
 #[test]
 fn test2() {
-    assert_eq!(Solution::convert("PAYPALISHIRING".to_string(), 4), "PINALSIGYAHRPI".to_string());
+    assert_eq!(Solution::convert("PAYPALISHIRING".to_string(), 4), "PINALSIGYAHRPI");
 }
 
 #[test]
 fn test3() {
-    assert_eq!(Solution::convert("A".to_string(), 1), "A".to_string());
+    assert_eq!(Solution::convert("A".to_string(), 1), "A");
 }