feat: add the solution of Array Partition I(561) with Java.

Author: Mukyu

README.md

@@ -58,7 +58,7 @@
 | [122][122-question] | [Best Time to Buy and Sell Stock II][122-tips]               | [✅][122-java] |              |                  |
 | [226][226-question] | [Invert Binary Tree][226-tips]                               | [✅][226-java] | [✅][226-js] |                  |
 | [543][543-question] | [Diameter of Binary Tree][543-tips]                          | [✅][543-java] |              |                  |
-| [561][561-question] | [Array Partition I][561-tips]                                |                | [✅][561-js] |                  |
+| [561][561-question] | [Array Partition I][561-tips]                                | [✅][561-java] | [✅][561-js] |                  |
 | [643][643-question] | [Maximum Average Subarray I][643-tips]                       |                | [✅][643-js] |                  |
 | [645][645-question] | [Set Mismatch][645-tips]                                     |                | [✅][645-js] |                  |
 | [653][653-question] | [Two Sum IV - Input is a BST][653-tips]                      |                | [✅][653-js] |                  |
@@ -443,6 +443,7 @@
 [226-java]: ./src/_226/Solution.java
 [543-java]: ./src/_543/Solution.java
 [554-java]: ./src/_554/Solution.java
+[561-java]: ./src/_561/Solution.java
 [728-java]: ./src/_728/Solution.java
 [771-java]: ./src/_771/Solution.java
 [804-java]: ./src/_804/Solution.java

src/_561/Solution.java

@@ -0,0 +1,31 @@
+package _561;
+
+public class Solution {
+
+    public int arrayPairSum(int[] nums) {
+        // 使用桶排序
+        // 创建map数组，map数组下标对应nums数组内的数字，存的是对应数字的数量
+        int[] map = new int[20001];
+        for (int num : nums) {
+            map[num + 10000]++;
+        }
+
+        // 遍历，取出奇数位的数并相加即可
+        int ans = 0;
+        boolean odd = false;
+        for (int i = 0; i < map.length; i++) {
+            while (map[i] != 0) {
+                if (odd = !odd) {
+                    ans += i - 10000;
+                }
+                map[i]--;
+            }
+        }
+        return ans;
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        System.out.println(solution.arrayPairSum(new int[]{1, 4, 3, 2}));
+    }
+}

tips/561/README.md

@@ -1,46 +1,60 @@
-[xxxx][title]
+[Array Partition I][title]
 
 ## Description
-// 抄题目
-
+Given an array of 2n integers, your task is to group these integers into n pairs of integer, say (a1, b1), (a2, b2), ..., (an, bn) which makes sum of min(ai, bi) for all i from 1 to n as large as possible.
 
 **Example:**
 
 ```
-// 抄Example
+Input: [1,4,3,2]
+
+Output: 4
+Explanation: n is 2, and the maximum sum of pairs is 4 = min(1, 2) + min(3, 4).
 ```
 
 **Note:**
-// Note
 
-**Tags:** // tags
+1. **n** is a positive integer, which is in the range of [1, 10000].
+2. All the integers in the array will be in the range of [-10000, 10000].
+
+**Tags:** [Array](https://leetcode.com/tag/array/)
 
 
 ## 思路 1
-// 贴一些关键代码,说一些解题思路
-// (同一种语言可以写多种思路，与某种语言思路相同的另一种语言的思路无须赘述，但可以把代码贴在后面) 
-```java
 
-```
-```javascript
+题目要求将所给数组两两分组为(a1, b1), (a2, b2), ..., (an, bn)，使得min(ai, bi)的和最大。
 
-```
+了解题目可知，对数组进行排序，取出奇数位的数字相加即可。题目限制数组中的数字范围为[-10000, 10000]，适合使用桶排序提高排序的速度。
 
-## 思路 2
-// 贴一些关键代码,说一些解题思路
 ```java
-
-```
-
-## 思路 3
-// 贴一些关键代码,说一些解题思路
-```kotlin
-
+public class Solution {
+    public int arrayPairSum(int[] nums) {
+        // 使用桶排序
+        // 创建map数组，map数组下标对应nums数组内的数字，存的是对应数字的数量
+        int[] map = new int[20001];
+        for (int num : nums) {
+            map[num + 10000]++;
+        }
+
+        // 遍历，取出奇数位的数并相加即可
+        int ans = 0;
+        boolean odd = false;
+        for (int i = 0; i < map.length; i++) {
+            while (map[i] != 0) {
+                if (odd = !odd) {
+                    ans += i - 10000;
+                }
+                map[i]--;
+            }
+        }
+        return ans;
+    }
+}
 ```
 
 ## 结语
-   
+
 如果你同我们一样热爱数据结构、算法、LeetCode，可以关注我们 GitHub 上的 LeetCode 题解：[LeetCode-Solution][ls]
 
-[title]: https://leetcode.com/problems/xxxx
+[title]: https://leetcode.com/problems/array-partition-i/description/
 [ls]: https://github.com/SDE603/LeetCode-Solution