快速排序&归并排序

他们思想的共同之处:分治
第一次看别人写的详细快排,归并细节,可能会感觉内容很多,很难学.小生建议:看一遍详解可以但不要指望一次就搞懂全部,搞懂基本原理就行,代码也不要指望第一次就能记住全部,先将代码会写,第一次写代码肯定会很困难.然后今后做题的时候肯定会遇到,今后重复重复+重复,就行了.
功夫是靠时间磨炼出来的,欲速则不达.

package Month3;
import java.util.*;
public class Demo231412 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int[] num = new int[] {5,4,2,8,7,9};
		mergeSort(num,0,num.length-1);
		for(int i=0;i<num.length;i++) {
			System.out.print(num[i]);
		}
	}
	//归并排序:
	static void mergeSort(int[] num,int left,int right) {
		if(left >= right) return;
		int mid = left+(right - left)/2;
//		mergeSort(num,left,mid-1);//这里有细节的,不能写成mid -1,会死递归,
		//如果写成mid -1,会导致right那一行死递归.
		mergeSort(num,left,mid);
		mergeSort(num,mid+1,right);
		
		int i = left,j = mid+1;
		int [] a = new int[num.length];
		int k = left;
		while(i<=mid && j<=right) {
			if(num[i]<=num[j]) {
				a[k++] = num[i++];
			}else {
				a[k++] = num[j++];
			}
		}
		while(i<=mid) a[k++] = num[i++];
		while(j<=right) a[k++] = num[j++];
		for(int x = left;x<=right;x++) {
			num[x] = a[x];
		}
	}
	//快速排序:
	static void qSort(int[] num,int left,int right) {
		
		if(left>=right) return;
		
		int i = left,j = right,cur = left;
		int key = num[left];
		while(cur<=j) {
			if(num[cur]<key) {
				int tmp = num[cur];
				num[cur] = num[i];
				num[i] = tmp;
				i++;
				cur++;
			}else if(num[cur]>key) {
				int tmp = num[cur];
				num[cur] = num[j];
				num[j] = tmp;
				j--;
			}else {
				cur++;
			}
		}
		qSort(num,left,i-1);
		qSort(num,j+1,right);
	}
}