[CareerCup] 7.7 The Number with Only Prime Factors 只有質數因子的數字

 

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();
    }
};