归并排序

package Sort;

import java.util.Arrays;
/*归并排序  稳定排序
时间复杂度: O(n*log n)
空间复杂度: O(n)
* */
public class mergeSort {
    public static void sort(int[] arr) {
        sortHelper(arr, 0, arr.length);
    }

    private static void sortHelper(int[] arr, int low, int high) {
        if (high - low <= 1) {
            return;
        }
        int mid = (low + high) / 2;
        sortHelper(arr, low, mid);
        sortHelper(arr, mid, high);
        merge(arr, low, mid, high);
    }

    private static void merge(int[] arr, int low, int mid, int high) {
        int cur1 = low;
        int cur2 = mid;
        int[] output = new int[high - low];
        int outputIndex = 0;

        while (cur1 < mid && cur2 < high) {
            // 此处必须是 <= , 不能是 < .
            // 如果是 < 是无法保证稳定性的.
            if (arr[cur1] <= arr[cur2]) {
                // 把 cur1 位置的元素插入到 output 中
                output[outputIndex] = arr[cur1];
                cur1++;
                outputIndex++;
            } else {
                output[outputIndex] = arr[cur2];
                cur2++;
                outputIndex++;
            }
        }
        while (cur1 < mid) {
            output[outputIndex] = arr[cur1];
            cur1++;
            outputIndex++;
        }
        while (cur2 < high) {
            output[outputIndex] = arr[cur2];
            cur2++;
            outputIndex++;
        }
        // 最后一步要把数据从临时空间中拷贝回原来的数组中.
        for (int i = 0; i < high - low; i++) {
            arr[low + i] = output[i];
        }
    }

    public static void main(String[] args) {
        int[] arr = {9,5,2,7,3,6,8};
        System.out.println(Arrays.toString(arr));
        sort(arr);
        System.out.println(Arrays.toString(arr));;
    }
}