归并排序

核心：将两个有序对数组归并成一个更大的有序数组。通常做法为递归排序，并将两个不同的有序数组归并到第三个数组中。

先来看看动图，归并排序是一种典型的分治应用。

public class MergeSort {public static void main(String[] args) {int unsortedArray[] = new int[]{6, 5, 3, 1, 8, 7, 2, 4};mergeSort(unsortedArray);System.out.println("After sort: ");for (int item : unsortedArray) {System.out.print(item + " ");}}private static void merge(int[] array, int low, int mid, int high) {int[] helper = new int[array.length];// copy array to helperfor (int k = low; k <= high; k++) {helper[k] = array[k];}// merge array[low...mid] and array[mid + 1...high]int i = low, j = mid + 1;for (int k = low; k <= high; k++) {// k means current locationif (i > mid) {// no item in left partarray[k] = helper[j];j++;} else if (j > high) {// no item in right partarray[k] = helper[i];i++;} else if (helper[i] > helper[j]) {// get smaller item in the right sidearray[k] = helper[j];j++;} else {// get smaller item in the left sidearray[k] = helper[i];i++;}}}public static void sort(int[] array, int low, int high) {if (high <= low) return;int mid = low + (high - low) / 2;sort(array, low, mid);sort(array, mid + 1, high);merge(array, low, mid, high);for (int item : array) {System.out.print(item + " ");}System.out.println();}public static void mergeSort(int[] array) {sort(array, 0, array.length - 1);}
}