From cd919ae432cdd9dff853c49e1d62b93e9d4bf5d6 Mon Sep 17 00:00:00 2001
From: Joshua <47053655+Joshua-Lu@users.noreply.github.com>
Date: Thu, 13 May 2021 23:03:45 +0800
Subject: [PATCH] =?UTF-8?q?Update=200216.=E7=BB=84=E5=90=88=E6=80=BB?=
 =?UTF-8?q?=E5=92=8CIII.md?=
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添加 0216.组合总和III Java版本
---
 problems/0216.组合总和III.md | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/problems/0216.组合总和III.md b/problems/0216.组合总和III.md
index 11a8eb8f..38af4c86 100644
--- a/problems/0216.组合总和III.md
+++ b/problems/0216.组合总和III.md
@@ -227,7 +227,38 @@ public:
 
 
 Java：
+```java
+class Solution {
+    List<List<Integer>> res = new ArrayList<>();
+    List<Integer> list = new ArrayList<>();
 
+    public List<List<Integer>> combinationSum3(int k, int n) {
+        res.clear();
+        list.clear();
+        backtracking(k, n, 9);
+        return res;
+    }
+
+    private void backtracking(int k, int n, int maxNum) {
+        if (k == 0 && n == 0) {
+            res.add(new ArrayList<>(list));
+            return;
+        }
+
+        // 因为不能重复，并且单个数字最大值是maxNum，所以sum最大值为
+        // （maxNum + (maxNum - 1) + ... + (maxNum - k + 1)） == k * maxNum - k*(k - 1) / 2
+        if (maxNum == 0
+                || n > k * maxNum - k * (k - 1) / 2
+                || n < (1 + k) * k / 2) {
+            return;
+        }
+        list.add(maxNum);
+        backtracking(k - 1, n - maxNum, maxNum - 1);
+        list.remove(list.size() - 1);
+        backtracking(k, n, maxNum - 1);
+    }
+}
+```
 
 Python：