diff --git a/src/solution/mod.rs b/src/solution/mod.rs
index 1c261c7..18fdf00 100644
--- a/src/solution/mod.rs
+++ b/src/solution/mod.rs
@@ -1 +1,3 @@
 mod s0001_two_sum;
+mod s0009_palindrome_number;
+mod s0020_valid_parentheses;
diff --git a/src/solution/s0009_palindrome_number.rs b/src/solution/s0009_palindrome_number.rs
new file mode 100644
index 0000000..c6b55ad
--- /dev/null
+++ b/src/solution/s0009_palindrome_number.rs
@@ -0,0 +1,74 @@
+/**
+ * [9] Palindrome Number
+ *
+ * Given an integer x, return true if x is a <span data-keyword="palindrome-integer">palindrome</span>, and false otherwise.
+ *  
+ * <strong class="example">Example 1:
+ * 
+ * Input: x = 121
+ * Output: true
+ * Explanation: 121 reads as 121 from left to right and from right to left.
+ * 
+ * <strong class="example">Example 2:
+ * 
+ * Input: x = -121
+ * Output: false
+ * Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.
+ * 
+ * <strong class="example">Example 3:
+ * 
+ * Input: x = 10
+ * Output: false
+ * Explanation: Reads 01 from right to left. Therefore it is not a palindrome.
+ * 
+ *  
+ * Constraints:
+ * 
+ * 	-2^31 <= x <= 2^31 - 1
+ * 
+ *  
+ * Follow up: Could you solve it without converting the integer to a string?
+ */
+pub struct Solution {}
+
+// problem: https://leetcode.cn/problems/palindrome-number/
+// discuss: https://leetcode.cn/problems/palindrome-number/discuss/?currentPage=1&orderBy=most_votes&query=
+
+// submission codes start here
+
+impl Solution {
+    pub fn is_palindrome(x: i32) -> bool {
+        if x == 0 {
+            return true
+        }
+
+        if x < 0 || x % 10 == 0 {
+            return false;
+        }
+
+        let mut reverse_half = x % 10;
+        let mut x = x / 10;
+
+        while x > reverse_half {
+            reverse_half = reverse_half * 10 + x % 10;
+            x = x / 10;
+        }
+
+        x == reverse_half || x == reverse_half / 10
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_9() {
+        assert_eq!(true, Solution::is_palindrome(121));
+        assert_eq!(false, Solution::is_palindrome(123));
+        assert_eq!(false, Solution::is_palindrome(10));
+        assert_eq!(true, Solution::is_palindrome(0));
+    }
+}
diff --git a/src/solution/s0020_valid_parentheses.rs b/src/solution/s0020_valid_parentheses.rs
new file mode 100644
index 0000000..d1bef4a
--- /dev/null
+++ b/src/solution/s0020_valid_parentheses.rs
@@ -0,0 +1,88 @@
+/**
+ * [20] Valid Parentheses
+ *
+ * Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
+ * An input string is valid if:
+ * <ol>
+ * 	Open brackets must be closed by the same type of brackets.
+ * 	Open brackets must be closed in the correct order.
+ * 	Every close bracket has a corresponding open bracket of the same type.
+ * </ol>
+ *  
+ * <strong class="example">Example 1:
+ * 
+ * Input: s = "()"
+ * Output: true
+ * 
+ * <strong class="example">Example 2:
+ * 
+ * Input: s = "()[]{}"
+ * Output: true
+ * 
+ * <strong class="example">Example 3:
+ * 
+ * Input: s = "(]"
+ * Output: false
+ * 
+ *  
+ * Constraints:
+ * 
+ * 	1 <= s.length <= 10^4
+ * 	s consists of parentheses only '()[]{}'.
+ * 
+ */
+pub struct Solution {}
+
+// problem: https://leetcode.cn/problems/valid-parentheses/
+// discuss: https://leetcode.cn/problems/valid-parentheses/discuss/?currentPage=1&orderBy=most_votes&query=
+
+// submission codes start here
+
+impl Solution {
+    pub fn is_valid(s: String) -> bool {
+        let left = vec!['(', '[', '{'];
+        let right = vec![')', ']', '}'];
+
+        let mut stack = Vec::with_capacity(s.len());
+
+        for c in s.chars() {
+            if left.contains(&c) {
+                stack.push(c)
+            }
+            else if right.contains(&c) {
+                let target = match c {
+                    ')' => '(',
+                    ']' => '[',
+                    '}' => '{',
+                    _ => return false
+                };
+
+                if stack.ends_with(&[target]) {
+                    stack.pop();
+                }
+                else {
+                    return false
+                }
+            } else {
+                return false
+            }
+        }
+
+        stack.is_empty()
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_20() {
+        assert_eq!(true, Solution::is_valid(String::from("()")));
+        assert_eq!(true, Solution::is_valid(String::from("()[]{}")));
+        assert_eq!(false, Solution::is_valid(String::from("(]")));
+        assert_eq!(false, Solution::is_valid(String::from("(")));
+    }
+}