CS61B学习笔记——归并排序

归并排序算法思路

如果数组长度为1，return将数组对半分，对两部分分别进行归并排序将上一步中归并排序后的数组合并起来

算法实现
    java

public class MergeSort {

    
    public int[] MergeSort(int[] a) {
        int size = a.length;
        if (size <= 1) {
            return a;
        } else {
            int halfSize = size / 2;
            int[] x = new int[halfSize];
            int[] y = new int[size - halfSize];
            System.arraycopy(a, 0, x, 0, halfSize);
            System.arraycopy(a, halfSize, y, 0, size-halfSize);
            int[] xSorted = MergeSort(x);
            int[] ySorted = MergeSort(y);
            return merge(xSorted, ySorted);
        }
    }

    
    private int[] merge(int[] x, int[] y) {
        if (x.length == 0) {
            return y;
        } else if (y.length == 0) {
            return x;
        } else {
            if (mergeCompare(x, y)) {
                return mergeHelper(x, y);
            } else {
                return mergeHelper(y, x);
            }
        }
    }

    
    private boolean mergeCompare(int[] x, int[] y) {
        return x[0] < y[0];
    }

    private int[] mergeHelper(int[] x, int[] y) {
        int[] res = new int[x.length + y.length];
        res[0] = x[0];
        int[] xNew = new int[x.length - 1];
        System.arraycopy(x, 1, xNew, 0, x.length - 1);
        int[] nums = merge(xNew, y);
        System.arraycopy(nums, 0, res, 1, x.length + y.length - 1);
        return res;
    }

    
    public static void main(String[] args) {
        MergeSort test = new MergeSort();
        int[] a = new int[]{1, -3, 6, 9, 22, -4, 3};
        int[] b = test.MergeSort(a);
        for (int i = 0; i < b.length; i++) {
            System.out.println(b[i]);
        }
    }
}