From 91d6940619835889565969f8fc893d1a22b7439f Mon Sep 17 00:00:00 2001
From: ehlxr <ehlxr.me@gmail.com>
Date: Sun, 4 Oct 2020 22:12:05 +0800
Subject: [PATCH] add merge sort

---
 src/main/java/me/ehlxr/sort/MergeSort.java | 69 ++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 src/main/java/me/ehlxr/sort/MergeSort.java

diff --git a/src/main/java/me/ehlxr/sort/MergeSort.java b/src/main/java/me/ehlxr/sort/MergeSort.java
new file mode 100644
index 0000000..dca833b
--- /dev/null
+++ b/src/main/java/me/ehlxr/sort/MergeSort.java
@@ -0,0 +1,69 @@
+package me.ehlxr.sort;
+
+import java.util.Arrays;
+
+/**
+ * 归并排序
+ * <p>
+ * 归并排序算法是将两个（或两个以上）有序表合并成一个新的有序表，即把待排序序列分为若干个子序列，
+ * 每个子序列是有序的。然后再把有序子序列合并为整体有序序列。
+ *
+ * @author ehlxr
+ * @since 2020-10-04 21:09.
+ */
+public class MergeSort {
+    public static void main(String[] args) {
+        int[] arr = {4, 9, 1, 8, 6, 2};
+        // merge(new int[]{1, 4, 9}, new int[]{2, 6, 8});
+        System.out.println(Arrays.toString(sort(arr)));
+    }
+
+    /**
+     * ①. 将序列每相邻两个数字进行归并操作，形成 floor (n/2) 个序列，排序后每个序列包含两个元素；
+     * ②. 将上述序列再次归并，形成 floor (n/4) 个序列，每个序列包含四个元素；
+     * ③. 重复步骤②，直到所有元素排序完毕
+     */
+    public static int[] sort(int[] arr) {
+        if (arr.length <= 1) {
+            return arr;
+        }
+        // arr.length/2
+        int num = arr.length >> 1;
+        int[] left = Arrays.copyOfRange(arr, 0, num);
+        int[] right = Arrays.copyOfRange(arr, num, arr.length);
+        return merge(sort(left), sort(right));
+    }
+
+    /**
+     * 将两个有序的数组进行归并排序操作
+     */
+    public static int[] merge(int[] arr1, int[] arr2) {
+        int l1 = 0;
+        int l2 = 0;
+        int i = 0;
+
+        int[] r = new int[arr1.length + arr2.length];
+        while (l1 < arr1.length || l2 < arr2.length) {
+            if (l1 >= arr1.length) {
+                r[i++] = arr2[l2];
+                l2++;
+                continue;
+            }
+
+            if (l2 >= arr2.length) {
+                r[i++] = arr1[l1];
+                l1++;
+                continue;
+            }
+
+            if (arr1[l1] < arr2[l2]) {
+                r[i++] = arr1[l1];
+                l1++;
+            } else {
+                r[i++] = arr2[l2];
+                l2++;
+            }
+        }
+        return r;
+    }
+}