本题最优时间复杂度为O(logN)

$\Sigma_{i=1} {i^2}$ = n(n+1)(2n+1)/6

结合二分可达到log级别复杂度

AC代码

#coding: utf-8
n:int = int(input())
def judge(x: int):
    return x*(x + 1)*(2 * x + 1) <= 6 * n
left = 0
right = n+1
while (right - left > 1):
    mid = int((left + right) / 2)
    if (judge(mid)):
        left = mid
    else:
        right = mid
print("%d\n" % (left))