求n階方陣對角線之和

      import java.util.*;  
       



public class Fangzhen {  
  
    public static void main(String[] args) {  
        // TODO Auto-generated method stub  
        int n = 0;  
        Scanner num = new Scanner(System.in);  
        System.out.println("請輸入階數：");  
         n = num.nextInt();  
            int[][] a =new int[n][n];  
        Scanner reader = new Scanner(System.in);  
        System.out.println("請輸入元素：");  
        for(int i = 0;i<n;i++){  
            for(int j = 0;j<n;j++){  
                a[i][j] = reader.nextInt();  
            }  
              
        }  
            int nm1,nm2,sum1 = 0,sum2 = 0,sum=0;  
            for(int i = 0;i<n;i++){  
                nm1 = a[i][i];  
                sum1 = sum1 + nm1;  
            }  
            for(int i = 0,j=n-1;i<n;i++,j--){  
                nm2 = a[i][j];  
                sum2 = sum2+nm2;  
            }  
            sum = sum1+sum2;  
            System.out.println("對角線上元素之和是"+sum);  
        }