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package com.yangchd.sort;

/**

* @author yangchd 2018/5/29

*

* 归并排序

* 1.选择相邻两个数组成一个有序序列。

* 2.选择相邻的两个有序序列组成一个有序序列。

* 3.重复第二步，直到全部组成一个有序序列。

*/

public class MergeSort {

public void mergeSort(int[] a) {

mergeSort(a, 0, a.length - 1);

}

private void mergeSort(int[] a, int low, int high) {

int mid = (low + high) / 2;

if (low < high) {

// 左边

mergeSort(a, low, mid);

// 右边

mergeSort(a, mid + 1, high);

// 左右归并

merge(a, low, mid, high);

}

}

private void merge(int[] a, int low, int mid, int high) {

int[] temp = new int[high - low + 1];

// 左指针

int i = low;

// 右指针

int j = mid + 1;

int k = 0;

// 把较小的数先移到新数组中

while (i <= mid && j <= high) {

if (a[i] < a[j]) {

temp[k++] = a[i++];

} else {

temp[k++] = a[j++];

}

}

// 把左边剩余的数移入数组

while (i <= mid) {

temp[k++] = a[i++];

}

// 把右边边剩余的数移入数组

while (j <= high) {

temp[k++] = a[j++];

}

// 把新数组中的数覆盖旧数组

System.arraycopy(temp, 0, a, low, temp.length);

}

}