题解 | 上三角矩阵判定

n = int(input())
Flag = True
for i in range(n):
    row = list(map(int,input().split()))
    for j in range(i):
        if row[j] != 0:
            Flag = False
            break
        else:
            pass
if Flag == True:
    print("YES")
else:
    print("NO")