题解 | 上三角矩阵判定

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int a[][] = new int[n][n];
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                a[i][j] = in.nextInt();
            }
        }
        boolean flag = true;//用flag去判断
        for(int i = 0; i < n; i++){
            for(int j = 0; j < i; j++){
                if(a[i][j] != 0){
                    flag = false;
                    break;
                }
            }
        }
        if(flag == true){
            System.out.print("YES");
        }else{
            System.out.print("NO");
        }
    }
}