Update 0039.组合总和.md

problems/0039.组合总和.md

@@ -236,7 +236,39 @@ public:
 
 
 Java：
+```Java
+class Solution {
+    List<List<Integer>> lists = new ArrayList<>();
+    Deque<Integer> deque = new LinkedList<>();
 
+    public List<List<Integer>> combinationSum3(int k, int n) {
+        int[] arr = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9};
+        backTracking(arr, n, k, 0);
+        return lists;
+    }
+
+    public void backTracking(int[] arr, int n, int k, int startIndex) {
+        //如果 n 小于0，没必要继续本次递归，已经不符合要求了
+        if (n < 0) {
+            return;
+        }
+        if (deque.size() == k) {
+            if (n == 0) {
+                lists.add(new ArrayList(deque));
+            }
+            return;
+        }
+        for (int i = startIndex; i < arr.length - (k - deque.size()) + 1; i++) {
+            deque.push(arr[i]);
+            //减去当前元素
+            n -= arr[i];
+            backTracking(arr, n, k, i + 1);
+            //恢复n
+            n += deque.pop();
+        }
+    }
+}
+```
 
 Python：