From fb91b760eb6fa0583210ca260b77f71418787ab8 Mon Sep 17 00:00:00 2001
From: hk27xing <244798299@qq.com>
Date: Sat, 29 May 2021 16:13:32 +0800
Subject: [PATCH] =?UTF-8?q?=E7=BA=A0=E6=AD=A339.=E7=BB=84=E5=90=88?=
 =?UTF-8?q?=E6=80=BB=E5=92=8C=20Java=E7=89=88=E6=9C=AC=E4=BB=A3=E7=A0=81?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 problems/0039.组合总和.md | 40 +++++++++++++++--------------------
 1 file changed, 17 insertions(+), 23 deletions(-)

diff --git a/problems/0039.组合总和.md b/problems/0039.组合总和.md
index ab118ee0..a83e42f4 100644
--- a/problems/0039.组合总和.md
+++ b/problems/0039.组合总和.md
@@ -237,34 +237,28 @@ public:
 
 Java：
 ```Java
+// 剪枝优化
 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 List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> res = new ArrayList<>();
+        Arrays.sort(candidates); // 先进行排序
+        backtracking(res, new ArrayList<>(), candidates, target, 0, 0);
+        return res;
     }
 
-    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) {
+    public void backtracking(List<List<Integer>> res, List<Integer> path, int[] candidates, int target, int sum, int idx) {
+        // 找到了数字和为 target 的组合
+        if (sum == target) {
+            res.add(new ArrayList<>(path));
             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();
+
+        for (int i = idx; i < candidates.length; i++) {
+            // 如果 sum + candidates[i] > target 就终止遍历
+            if (sum + candidates[i] > target) break;
+            path.add(candidates[i]);
+            backtracking(res, path, candidates, target, sum + candidates[i], i);
+            path.remove(path.size() - 1); // 回溯，移除路径 path 最后一个元素
         }
     }
 }