题解 | #KiKi判断上三角矩阵#
KiKi判断上三角矩阵
https://www.nowcoder.com/practice/9a6786c28cdb45f9b991685f867b5d08
n = int(input())
li = []
flag = 0
for i in range(n):
li += list(map(int, input().split()[0:i])) #获取所有对角线以下的数字
for j in li:
if j != 0:
flag = 1
print("NO")
break
if flag == 0:
print("YES")