题解 | 上三角矩阵判定

n = int(input())
cou = 0  
for i in range(n):  
    a = list(map(int, input().split()))  
    if i == 0:
        cou += 1
    else:
        count = 0
        for j in range(i):  
            if a[j] == 0:
                count += 1
        if count == i:
            cou += 1
if cou == n:
    print("YES")
else:
    print("NO")