[CareerCup] 7.7 The Number with Only Prime Factors 只有质数因子的数字

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();
    }
};