diff --git a/CombinationSum.java b/CombinationSum.java
new file mode 100644
index 00000000..e55fe4ed
--- /dev/null
+++ b/CombinationSum.java
@@ -0,0 +1,76 @@
+// 0-1 recursion or choose/notChoose recursion
+class Solution {
+
+    List<List<Integer>> result;
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        this.result = new ArrayList<>();
+        helper(candidates, target, new ArrayList<>(), 0);
+        return result;
+    }
+
+    private void helper (int[] candidates, int target, List<Integer> path, int index) {
+
+        // base
+        if (target == 0) {
+            result.add(new ArrayList<>(path)); // copy
+            return;
+        }
+
+        if (target < 0 || candidates.length == index) {
+            return;
+        }
+
+        // logic
+
+        // not choose
+        helper(candidates, target, path, index + 1);
+
+        // choose
+        path.add(candidates[index]);
+        helper(candidates, target - candidates[index], path, index);
+        path.remove(path.size() - 1);
+    }
+}
+
+// for loop based recursion
+class Solution {
+
+    List<List<Integer>> result;
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        this.result = new ArrayList<>();
+        helper(candidates, target, new ArrayList<>(), 0);
+        return result;
+    }
+
+    private void helper (int[] candidates, int target, List<Integer> path, int pivot) {
+
+        // base
+        if (target == 0) {
+            result.add(new ArrayList<>(path)); // copy
+            return;
+        }
+
+        if (target < 0) {
+            return;
+        }
+
+        // logic
+        for (int i = pivot; i < candidates.length; i++) {
+
+            //action
+            path.add(candidates[i]);
+
+            //recurse
+            helper(candidates, target - candidates[i], path, i);
+
+            //backtrack
+            path.remove(path.size() - 1);
+        }
+        
+    }
+}
+
+// Time = O(2^(M+N))
+// Space = O(N)
\ No newline at end of file
diff --git a/Subsets.java b/Subsets.java
new file mode 100644
index 00000000..493d0a74
--- /dev/null
+++ b/Subsets.java
@@ -0,0 +1,80 @@
+// 0-1 recursion
+class Solution {
+
+    List<List<Integer>> result;
+
+    public List<List<Integer>> subsets(int[] nums) {
+        this.result = new ArrayList<>();
+        helper(nums, new ArrayList<>(), 0);
+        return result;
+    }
+
+    private void helper (int nums[], List<Integer> path, int index) {
+
+        // base
+        if (index == nums.length) {
+            result.add(new ArrayList<>(path)); // deep copy
+            return;
+        }
+
+        //not choose
+        helper(nums, path, index + 1);
+
+        //choose
+        path.add(nums[index]);
+        helper(nums, path, index + 1);
+
+        //backtrack
+        path.remove(path.size() - 1);
+    }
+}
+
+// for lopp recursion
+class Solution {
+
+    List<List<Integer>> result;
+
+    public List<List<Integer>> subsets(int[] nums) {
+        this.result = new ArrayList<>();
+        helper(nums, new ArrayList<>(), 0);
+        return result;
+    }
+
+    private void helper (int nums[], List<Integer> path, int pivot) {
+    
+        result.add(new ArrayList<>(path)); // deep copy
+
+        for(int i = pivot; i < nums.length; i++) {
+            path.add(nums[i]);
+            helper(nums, path, i + 1);
+            //backtrack
+            path.remove(path.size() - 1);
+        }
+    }
+}
+
+// Start with the empty subset in the result list.
+//For each number, copy all current subsets and append the number to create new subsets.
+class Solution {
+
+    public List<List<Integer>> subsets(int[] nums) {
+
+        List<List<Integer>> result = new ArrayList<>();
+        result.add(new ArrayList<>());
+
+        for(int i = 0; i < nums.length; i++) {
+            int size = result.size();
+            for(int j = 0; j < size; j++) {
+                List<Integer> temp = new ArrayList<>(result.get(j));
+                temp.add(nums[i]);
+                result.add(temp);
+            }
+        }
+        
+        return result;
+    }
+}
+
+
+// Time = O(N*2^N)
+// Space = O(N)
\ No newline at end of file