杨辉三角形，最后语句超出索引范围

Question

a = [1]*10
for i in range(len(a)):
    a[i] = [1]*(i+1)
for x in range(len(a)):
    print(a[x])
    for y in range(len(a)):
        if y<x:
            a[x+1][y+1] = a[x][y]+a[x][y+1

最后的语句为什么会超出索引范围啊
Traceback (most recent call last):
File "D:/python/p25杨辉三角形.py", line 24, in
a[x+1][y+1] = a[x][y]+a[x][y+1]
IndexError: list index out of range

Answer 1

y取值最大值为长度减去1，那么y+1就等于a的长度，作为下标就越界了。因为下标范围是0-长度减去1