题目描述
编写一个程序，找出第 n 个丑数，并给出该程序的时间复杂度。丑数：质因数只包含2，3，5的正整数。n<1690。质因数：能整除给定正整数的质数。
def fun(num): if num<1: return 0
temp = 0
a2 = a3 = a5 =0
while len(temp)<num:
temp.append(min(temp[a2]*2,temp[a3]*3,temp[a5]*5))
while temp[a2]*2<=temp[-1]:
a2+=1
while temp[a3]*3<=temp[-1]:
a3+=1
while temp[a5]*5<=temp[-1]:
a5+=1
return temp[-1]
提示：
step1: temp = [1] , t2 = 0 , t3 = 0 , t5 = 0
step2: temp = [1,2] , t2 = 1 , t3 = 0 , t5 = 0
step3: temp = [1,2,3] , t2 = 1 , t3 = 1 , t5 = 0
step4: temp = [1,2,3,4] , t2 = 2 , t3 = 1 , t5 = 0
step5: temp = [1,2,3,4,5] , t2 = 2 , t3 = 1 , t5 = 1
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