java常用算法笔记

1.将一个10进制的c转换为n进制
String s=new BigInteger(c+"",10).toString(n);
2.
求一个解退出
System.exit(0)；
3.比较器
Arrays.sort(p,0,m,new Comparator<Point>(){//按照第0列排序
            public int compare(Point x,Point y){
                return x.a-y.a;
            }
                    });
4.翻转字符串
String t=new StringBuilder(s).reverse().toString();
5.队列
Queue<String> queue = new LinkedList<String>();
6.HashMap遍历
for(String s:dict.keySet()) {
	System.out.println(s+" is "+dict.get(s));
}
6.种子填充
void dfs(char[][] a, boolean[][] b, int i, int j,int n) 
	
	if(i<0||j<0||i>=n||j>=n) 
	        return;
	if(b[i][j]==true||a[i][j]!='@')
	        return;
	else 
	{
	        b[i][j]=true;
	        for(int dr=-1;dr<2;dr++) 
	        {
	                for(int dc=-1;dc<2;dc++) 
	                {
	                        if(dc==0&&dr==0)continue;
	                        else 
	                        {
	                                dfs(a,b,i+dr,j+dc,n);
	                        }
	                }
	        }
	}
	
	        
7.星期几
static int whatday(int y, int m, int d) 
{
        return (m==1||m==2)?
		(d+2*(m+12)+3*(m+13)/5+(y-1)+(y-1)/4-(y-1)/100+(y-1)/400)%7
		:(d+2*m+3*(m+1)/5+y+y/4-y/100+y/400)%7; 
 }
8.全排列
private static void dfs(int n, int k) {
		if(k==n) {
			for(int i=0;i<n;i++) {
				System.out.print(a[i]+" ");
			}
			System.out.println();
		}
		else {
			
			for(int i=0;i<n;i++) {
				int ok=1;
				for(int j=0;j<k;j++) {
					if(a[j]==i+1) {
						ok=0;
					}
				}
				if(ok==1) {
					a[k]=i+1;
					dfs(n,k+1);
				}
				
			}
		}
	}
9.欧几里得
private static int gcd(int a, int b) {
		// TODO Auto-generated method stub
		if(b==0)return a;
		else {
			return gcd(b,a%b);
		}
	}
10.快速幂
public static long ex(long n,long m) {
		long result=1;
		long pingfangshu=n;
		while(m!=0) {
			if((m&1)==1) result*=pingfangshu;
			pingfangshu=pingfangshu*pingfangshu;
			m>>=1;
		}
		return result;
		
	}
11.可重集全排列
public static void dfs(int n,int k) 
	{
		if(k==n) 
		{
			for(int i=0;i<n;i++)
				System.out.print(a[i]);
			System.out.println();
		}
		else {
			for(int i=0;i<n;i++) 
			{
				if(i==0||b[i-1]!=b[i]) 
				{
						int c1=0,c2=0;
						for(int j=0;j<k;j++)
							if(a[j]==b[i])c1++;
						for(int j=0;j<n;j++)
							if(b[i]==b[j])c2++;
						if(c1<c2) 
						{
							a[k]=b[i];
							dfs(n,k+1);
						}
				}
			}
		}
	}
12.过了某天是几月几日
Calendar cd=Calendar.getInstance();
	cd.set(y, m-1,d);
	cd.add(Calendar.DATE, k);
	String format=(new SimpleDateFormat("yyyy-MM-dd")).format(cd.getTime());
13.比较器
Arrays.sort(p,0,m,new Comparator<Point>() {
			public int compare(Point x,Point y) {
				return x.a-y.a;
			}
		});
14.Arraylist排序
Collections.sort(list);