diff --git a/CombinationSum.java b/CombinationSum.java
new file mode 100644
index 00000000..4df4ea2a
--- /dev/null
+++ b/CombinationSum.java
@@ -0,0 +1,62 @@
+// Time Complexity :O(2^(m+n))
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode :yes
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> answer = new ArrayList<>();
+        List<Integer> path = new ArrayList<>();
+        helper(candidates, target, 0,path,answer);
+        return answer;
+    }
+    private void helper(int[] candidates, int target, int i,List<Integer> path,List<List<Integer>> answer  ){
+        if(target < 0 || i == candidates.length) return;
+        if(target == 0){
+            answer.add(new ArrayList<>(path)); 
+            return;
+        }
+        //nochoose
+        helper(candidates, target, i+1, path, answer);
+        //choose
+        //action
+        path.add(candidates[i]);
+        //recurse
+        helper(candidates, target - candidates[i], i, path, answer);
+        //backtrack
+        path.remove(path.size()-1); 
+    }
+}
+
+
+
+
+// Time Complexity :O(2^(m+n))
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode :yes
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> answer = new ArrayList<>();
+        List<Integer> path = new ArrayList<>();
+        helper(candidates, target, 0, path, answer);
+        return answer;
+    }
+
+    private void helper(int[] candidates, int target, int pivot, List<Integer> path, List<List<Integer>> answer) {
+        if (target < 0 || pivot == candidates.length)
+            return;
+        if (target == 0) {
+            answer.add(new ArrayList<>(path));
+            return;
+        }
+        for (int i = pivot; i < candidates.length; i++) {
+            //action
+            path.add(candidates[i]);
+            //recurse
+            helper(candidates, target - candidates[i], i, path, answer);
+            //backtrack
+            path.remove(path.size() - 1);
+        }
+    }
+}
\ No newline at end of file
diff --git a/ExpressionAddOperators.java b/ExpressionAddOperators.java
new file mode 100644
index 00000000..5a327838
--- /dev/null
+++ b/ExpressionAddOperators.java
@@ -0,0 +1,39 @@
+// Time Complexity :O(4^n * n)
+// Space Complexity :O(n^2)
+// Did this code successfully run on Leetcode :yes
+//     | calc val             |   tail
+//   + | calc+curr            |   curr    
+//   - | calc-curr            |   -curr
+//   * | calc-tail+tail*curr  |  tail*curr
+
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+    public List<String> addOperators(String num, int target) {
+        List<String> answer = new ArrayList<>();
+        helper(num, target, 0, 0, 0, "",answer);
+        return answer;
+    }
+    private void helper(String num, int target, int pivot, long calc, long tail, String path, List<String> answer ) {
+        if (pivot == num.length()) {
+            if (calc == target) {
+                answer.add(path);
+            }
+        }
+        for (int i = pivot; i < num.length(); i++) {
+            if (num.charAt(pivot) == '0' && i != pivot) break; // preceding 0 case
+            long curr = Long.parseLong(num.substring(pivot, i + 1));
+            if (pivot == 0) {
+                helper(num, target, i + 1, curr, curr, path + curr, answer);
+            } else {
+                // +
+                helper(num, target, i + 1, calc + curr, curr, path + "+" + curr, answer);
+                // -
+                helper(num, target, i + 1, calc - curr, -curr, path + "-" + curr, answer);
+                // *
+                helper(num, target, i + 1, calc - tail + (tail * curr), tail * curr, path + "*" + curr, answer);
+            }
+        }
+    }
+}