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();
        in.nextLine();
        //同时判断右上三角和左上三角
        boolean flag_01 = true;
        boolean flag_02 = true;

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                //右上三角
                int temp = in.nextInt();
                if (i > j &&  temp != 0) {
                    flag_02 = false;
                }
                //左上三角
                if (i + j > n - 1 && temp != 0) {
                    flag_01 = false;
                }
            }
        }
        if (flag_02) {
            System.out.print("YES");
        } else if(flag_01){
            System.out.print("YES");
        }else{
            System.out.print("NO");
        }
    }
}