Before learning to merge and sort, please have a general look Eight sorting algorithms that programmers must master

(1) Basic ideas

Merge sorting is to merge two (or more) ordered tables into a new ordered table, that is, to divide the sequence to be sorted into several subsequences, each subsequence is ordered. Then the ordered subsequences are combined into the whole ordered sequences.

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(2) Code implementation

import java.util.Arrays;

public class Sort {

    public static void mergeSort(int[] array) {
        sort(array, 0, array.length - 1);
    }

    private static void sort(int[] data, int left, int right) {
        if (left < right) {
            // Find the middle index
            int center = (left + right) / 2;
            // Recurse the left array
            sort(data, left, center);
            // Recurse the right array
            sort(data, center + 1, right);
            // merge
            merge(data, left, center, right);
        }
    }

    private static void merge(int[] data, int left, int center, int right) {
        int[] tmpArr = new int[data.length];
        int centerNext = center + 1;
        // Index of record temporary array
        int tmp = left;
        int index = left;
        while (left <= center && centerNext <= right) {
            //Take the smallest of the two arrays and put it into the temporary array
            if (data[left] <= data[centerNext]) {
                tmpArr[tmp++] = data[left++];
            } else {
                tmpArr[tmp++] = data[centerNext++];
            }
        }

        // If there are remaining elements in the right array, put them in the temporary array in turn
        while (centerNext <= right) {
            tmpArr[tmp++] = data[centerNext++];
        }

        // If there are remaining elements in the left array, put them in the temporary array in turn
        while (left <= center) {
            tmpArr[tmp++] = data[left++];
        }

        // Copy the contents of the temporary array back to the original array
        while (index <= right) {
            data[index] = tmpArr[index++]; 
        }
    }

    public static void main(String[] args) {
        int[] arr = {57, 68, 59, 52, 72, 28, 96, 33};
        System.out.println("Original array: " + Arrays.toString(arr));  
        mergeSort(arr);
        System.out.println("Sorted array: " + Arrays.toString(arr));   
    }
}

Operation result:

> javac -encoding utf-8 Sort.java
> java Sort

Original array: [57, 68, 59, 52, 72, 28, 96, 33]
Sorted array: [28, 33, 52, 57, 59, 68, 72, 96]

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