归并排序

一、问题描述

对数组进行排序

二、方法

1. 每次对两个小数组进行排序
2. 将整个大数组一直切分成小数组

三、思想

分治。

四、code

 

package algorithm;

/**
 * Created by adrian.wu on 2019/2/14.
 */
public class MergeSort {
    public void merge(int[] nums, int s, int m, int e) {
        if (s >= e) return;

        //new sorted nums
        int[] nsn = new int[e - s + 1];

        //as is the start position of s, bs is the start position of b, c is count
        int c = 0, as = s, bs = m + 1;
        while (as <= m && bs <= e) {
            if (nums[as] <= nums[bs]) {
                nsn[c++] = nums[as++];
            } else {
                nsn[c++] = nums[bs++];
            }
        }

        while (as <= m) nsn[c++] = nums[as++];
        while (bs <= e) nsn[c++] = nums[bs++];

        for (int i = s; i <= e; i++) {
            nums[i] = nsn[i - s];
        }
    }

    public void mergeSort(int[] nums, int s, int e) {
        //next mid
        if (s >= e) return;
        int m = (e + s) / 2;
        mergeSort(nums, s, m);
        mergeSort(nums, m + 1, e);
        merge(nums, s, m, e);
    }
}