Add Solution.java to problems 0039

Author: huaxu1024

solution/0039.Combination Sum/Solution2.java

@@ -0,0 +1,24 @@
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> res = new ArrayList<>();
+        combinationSum(candidates, target, 0, new Stack<>(), res);
+        return res;
+    }
+
+    private void combinationSum(int[] candidates, int target, int index, Stack<Integer> stack, List<List<Integer>> res) {
+        if (target < 0) {
+            return;
+        }
+
+        if (target == 0) {
+            res.add(new ArrayList<>(stack));
+            return;
+        }
+
+        for (int i = index; i < candidates.length; i++) {
+            stack.push(candidates[i]);
+            combinationSum(candidates, target - candidates[i], i, stack, res);
+            stack.pop();
+        }
+    }
+}