杨辉三角的规律：
1、第n行有n个数字.
2、每一行的开始和结尾数字都为1。（用二维数组表示就是a[i][0]=1; a[i][j]=1(当i==j时)）
3、第n+1行的第i个数字等于第n行的i-1个数字加上第n行的i个数字。（用二维数组表示就是 a[i+1][j]=a[i][j-1]+a[i][j];）
方法一：

import java.util.Scanner;

public class Main {  
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        int [][] a = new int [n][n];
        for(int i = 0;i < a.length;i++) {
            for(int j = 0;j <= i;j++) {
                if(j == 0 || j == i) {
                    a[i][j] = 1;
                }
                else {
                    a[i][j] = a[i-1][j-1] + a[i-1][j];
                }
                System.out.print(a[i][j] + " ");
            }
            System.out.println();
        }
    }
}

方法二：

import java.util.Scanner;

public class Main {  
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        int[][] a = new int[n][];
        for(int i=0;i<a.length;i++) {
            a[i] = new int[i + 1];
        }
        for(int i = 0;i < a.length;i++) {
            for(int j = 0;j < a[i].length;j++) {
                if(j == 0 || i == j) {
                    a[i][j]=1;
                }else {
                    a[i][j]=a[i-1][j]+a[i-1][j-1];
                }
            }
        }
        for(int i = 0;i < a.length;i++) {
            for(int j = 0;j < a[i].length;j++) {
                System.out.print(a[i][j] + " ");
            }
            System.out.println();
        }
    }
}

方法三：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        for (int i = 0; i < n; i++) {
            int number = 1;
            System.out.format("%" + (n - i) * 2 + "s", "");
            for (int j = 0; j <= i; j++) {
                System.out.format("%4d", number);
                number = number * (i - j) / (j + 1);
            }
            System.out.println();
        }
    }
}