add solution for combination-sum

Author: Ujoonnee

combination-sum/Ujoonnee.java

@@ -0,0 +1,28 @@
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> output = new ArrayList<>();
+        Stack<Integer> nums = new Stack<>();
+        dfs(candidates, output, nums, target, 0, 0);
+
+        return output;
+    }
+
+    private void dfs(int[] candidates, List<List<Integer>> output, Stack<Integer> nums, int target, int start, int total) {
+        if (total > target) {
+            return;
+        }
+        if (total == target) {
+            output.add(new ArrayList<>(nums));
+        }
+        for (int i=start; i<candidates.length; i++) {
+            int num = candidates[i];
+            nums.push(num);
+            dfs(candidates, output, nums, target, i, total + num);
+            nums.pop();
+        }
+    }
+}