题解 | #KiKi判断上三角矩阵#

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[][] nums = new int[n][n];
        for(int i = 0;i < n;i++){
            for(int j = 0;j < n;j++){
                nums[i][j] = in.nextInt();
            }
        }
        boolean tag = true;
        for(int i = 0;i < n;i++){
            for(int j = i+1;j < n;j++){
                if(nums[j][i] != 0){
                    tag = false;
                    break;
                }
            }
            if(i+1 >= n){
                break;
            }
        }
        if(tag == true){
            System.out.println("YES");
        }else{
            System.out.println("NO");
        }
    }
}