diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index b35cd27..78345f3 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -167,4 +167,5 @@ mod p211_design_add_and_search_words_data_structure;
 mod p212_word_search_ii;
 mod p17_letter_combinations_of_a_phone_number;
 mod p77_combinations;
-mod p46_permutations;
\ No newline at end of file
+mod p46_permutations;
+mod p39_combination_sum;
\ No newline at end of file
diff --git a/src/problem/p39_combination_sum.rs b/src/problem/p39_combination_sum.rs
new file mode 100644
index 0000000..30282c7
--- /dev/null
+++ b/src/problem/p39_combination_sum.rs
@@ -0,0 +1,61 @@
+/**
+ * [39] Combination Sum
+ */
+pub struct Solution {}
+
+
+// submission codes start here
+use std::collections::HashSet;
+
+impl Solution {
+    pub fn combination_sum(candidates: Vec<i32>, target: i32) -> Vec<Vec<i32>> {
+        let mut result = HashSet::new();
+        let mut candidates = candidates;
+        candidates.sort();
+
+        let mut path = vec![];
+        Self::dfs(&candidates, target, &mut path, 0, &mut result);
+
+        result.iter().map(|x| x.clone()).collect()
+    }
+
+    fn dfs(candidates: &Vec<i32>, target: i32, path: &mut Vec<i32>, sum: i32, result: &mut HashSet<Vec<i32>>) {
+        if sum > target {
+            return;
+        }
+
+        if sum == target {
+            let mut p = path.clone();
+            p.sort();
+            result.insert(p);
+            return;
+        }
+
+        for &i in candidates {
+            if i > target - sum {
+                break;
+            }
+
+            path.push(i);
+            Self::dfs(candidates, target, path, sum + i, result);
+            path.pop();
+        }
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_39() {
+        let mut set = HashSet::new();
+        
+        set.insert(vec![1,2]);
+        set.insert(vec![1,2]);
+        
+        assert_eq!(set.len(), 1);
+    }
+}