7.7 Design an algorithm to find the kth number such that the only prime factors are 3,5, and 7.

這道題跟之前LeetCode的那道Ugly Number II 醜陋數之二基本沒有啥區別，具體講解可參見那篇，程式碼如下：

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();
    }
};