Added 039_Combination_Sum.java (qiyuangong#75)

* Added 039_Combination_Sum.java

Contributed by @green-study

Author: green-study

java/039_Combination_Sum.java

@@ -0,0 +1,29 @@
+//039_Combination_Sum
+class Solution {
+    List<List<Integer>> answer = new ArrayList<List<Integer>>();
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        int clen =candidates.length;
+        for (int i = 0; i < clen; i++) {
+            List<Integer> tlist = new ArrayList<Integer>();
+            tlist.add(candidates[i]);
+            backtracking(candidates, i, 1, (target - candidates[i]), tlist);
+        }
+        return answer;
+    }
+    private void backtracking(int[] candidates, int index, int tsize, int target, List<Integer> temp) {
+        if (target == 0) {
+            answer.add(new ArrayList(temp));
+            return;
+        }
+        
+        for (int i = index, len = candidates.length; i < len; i++) {
+            if (candidates[i] <= target) {
+                temp.add(candidates[i]);
+                backtracking(candidates, i, (tsize + 1), (target - candidates[i]), temp);
+                temp.remove(tsize);
+            }
+        }
+    }
+}
+