diff --git a/src/lib.rs b/src/lib.rs
index 4a2f141..9f8a9ce 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,3 +14,5 @@ pub mod p13_roman_to_integer;
 pub mod p14_longest_common_prefix;
 pub mod p15_3sum;
 pub mod p16_3sum_closest;
+pub mod p17_letter_combinations_of_a_phone_number;
+pub mod p18_4sum;
diff --git a/src/p17_letter_combinations_of_a_phone_number.rs b/src/p17_letter_combinations_of_a_phone_number.rs
new file mode 100644
index 0000000..0caf26b
--- /dev/null
+++ b/src/p17_letter_combinations_of_a_phone_number.rs
@@ -0,0 +1,46 @@
+pub struct Solution;
+impl Solution {
+    const LETTER_GROUPS: [&'static[char]; 8] = [
+        &['a','b','c'], &['d','e','f'], &['g','h','i'],
+        &['j','k','l'], &['m','n','o'], &['p','q','r','s'],
+        &['t','u','v'], &['w','x','y','z']
+    ];
+    
+    pub fn letter_combinations(digits: String) -> Vec<String> {
+        if digits.is_empty() { return vec![]; }
+        let mut result = vec!["".to_string()];
+        for b in digits.bytes() {
+            let group_index = b - b'0' - 2;
+            let group = Self::LETTER_GROUPS[group_index as usize];
+            let extend_size = result.len();
+            for _ in 0..group.len()-1 { result.extend_from_within(..extend_size); }
+            for (i, s) in result.iter_mut().enumerate() {
+                let index = i / extend_size;
+                s.push(group[index]);
+            }
+        }
+        return result;
+    }
+}
+
+#[test]
+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()
+    ]);
+}
+
+#[test]
+fn test2() {
+    assert_eq!(Solution::letter_combinations(String::new()), Vec::<String>::new());
+}
+
+#[test]
+fn test3() {
+    assert_eq!(Solution::letter_combinations("2".to_string()),
+        vec!["a".to_string(), "b".to_string(), "c".to_string()]);
+}
diff --git a/src/p18_4sum.rs b/src/p18_4sum.rs
new file mode 100644
index 0000000..67fe639
--- /dev/null
+++ b/src/p18_4sum.rs
@@ -0,0 +1,55 @@
+pub struct Solution;
+impl Solution {
+    pub fn four_sum(mut nums: Vec<i32>, target: i32) -> Vec<Vec<i32>> {
+        let mut result = vec![];
+        if nums.len() < 4 { return result; }
+        nums.sort();
+        for i in 0..nums.len()-3 {
+            if i > 0 && nums[i - 1] == nums[i] { continue; }
+            for j in i+1..nums.len()-2 {
+                if j > i + 1 && nums[j - 1] == nums[j] { continue; }
+                let (mut k, mut l) = (j + 1, nums.len() - 1);
+                while k < l {
+                    if k > j + 1 && nums[k - 1] == nums[k] {
+                        k += 1; continue;
+                    }
+                    if l < nums.len() - 1 && nums[l + 1] == nums[l] {
+                        l -= 1; continue;
+                    }
+                    // num is in range [-10^9; 10^9], possibility of overflow
+                    let Some(sum) = (nums[i] + nums[j]).checked_add(nums[k] + nums[l])
+                    else { k += 1; continue; };
+                    match (sum - target).signum() {
+                        -1 => { k += 1; },
+                        1 => { l -= 1; },
+                        0 => {
+                            result.push(vec![nums[i], nums[j], nums[k], nums[l]]);
+                            k += 1; l -= 1;
+                        },
+                        _ => ()
+                    }
+                }
+            }
+        }
+        return result;
+    }
+}
+
+#[test]
+fn test1() {
+    let mut actual = Solution::four_sum(vec![1,0,-1,0,-2,2], 0);
+    actual.iter_mut().for_each(|v| v.sort());
+    assert_eq!(actual, vec![
+        vec![-2,-1,1,2], vec![-2,0,0,2], vec![-1,0,0,1]
+    ]);
+}
+
+#[test]
+fn test2() {
+    assert_eq!(Solution::four_sum(vec![2,2,2,2,2], 8), vec![vec![2,2,2,2]]);
+}
+
+#[test]
+fn test3() {
+    assert_eq!(Solution::four_sum(vec![1000000000,1000000000,1000000000,1000000000], -294967296), Vec::<Vec<i32>>::new());
+}