Write a program to find the n-th ugly number.
Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. For example, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.
Note that 1 is typically treated as an ugly number.
如果通过判断每个数是不是Ugly number，时间复杂度将很高。这里可以采用判断K内有多少个素数这个问题的解法，稍加变通。
首先1是ugly number，下个数是其*2，*3，*5的最小值。
用i2来代表是第几次*2，i3代表第几次*3，i5代表第几次*5。
抓住第n+1次*2比第n次*2大这个原理，*2后位置+1。
1 class Solution { 2 public: 3 int nthUglyNumber(int n) { 4 vector<int> uglys; 5 uglys.push_back(1); 6 int i2=0,i3=0,i5=0; 7 for(int i=1;i<n;i++) 8 { 9 int min_next = min(min(uglys[i2]*2,uglys[i3]*3),uglys[i5]*5); 10 uglys.push_back(min_next); 11 if(min_next==uglys[i2]*2) i2++; 12 if(min_next==uglys[i3]*3) i3++; 13 if(min_next==uglys[i5]*5) i5++; 14 } 15 return uglys[n-1]; 16 } 17 };
原文：http://www.cnblogs.com/Sean-le/p/4767573.html