归并排序

归并排序

归并排序是一种概念上最简单的排序算法，与快速排序一样，归并排序也是基于分治法的。归并排序将待排序的元素序列分成两个长度相等的子序列，为每一个子序列排序，然后再将他们合并成一个子序列。合并两个子序列的过程也就是两路归并。

public class MergeSort {
    public static void main(String[] args) {
        int[] arr = {1, 4, 6, 3, 2, 7, 4, 9, 20, 10};
        int[] copy = new int[10];
        mergeArr(arr, copy, 0, arr.length - 1);
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }

    public static void mergeArr(int[] arr, int[] copy, int low, int high) {
        if (low == high)
            return;
        int mid = (low + high) / 2;
        mergeArr(arr, copy, low, mid);
        mergeArr(arr, copy, mid + 1, high);
        int i = mid;
        int j = high;
        int copyIndex = high;
        while (i >= low && j > mid) {
            if (arr[i] > arr[j]) {
                copy[copyIndex--] = arr[i--];
            } else
                copy[copyIndex--] = arr[j--];
        }
        for (; i >= low; i--)
            copy[copyIndex--] = arr[i];
        for (; j > mid; j--)
            copy[copyIndex--] = arr[j];
        for (int k = low; k <= high; k++) {
            arr[k] = copy[k];
        }
    }
}