题解 | 上三角矩阵判定

n = int(input())

list_a = []

for _ in range(n):
   a = list(map(int, input().split()))
   list_a.append(a)

for i in range(n):
    for j in range(i):
        if list_a[i][j] != 0:
            print('NO')
            exit()

print('YES')