7.7 Design an algorithm to find the kth number such that the only prime factors are 3,5, and 7.
这道题跟之前LeetCode的那道Ugly Number II 丑陋数之二基本没有啥区别，具体讲解可参见那篇，代码如下：
class Solution { public: int getKthMagicNumber(int k) { vector<int> res(1, 1); int i3 = 0, i5 = 0, i7 = 0; while (res.size() < k) { int m3 = res[i3] * 3, m5 = res[i5] * 5, m7 = res[i7] * 7; int mn = min(m3, min(m5, m7)); if (mn == m3) ++i3; if (mn == m5) ++i5; if (mn == m7) ++i7; res.push_back(mn); } return res.back(); } };
[CareerCup] 7.7 The Number with Only Prime Factors 只有质数因子的数字
原文：http://www.cnblogs.com/grandyang/p/4780868.html