分析：分别设三个指针，表明乘以3。5，7会大于当前丑数的index

代码：

class Solution {

public:

    /*

     * @param k: The number k.

     * @return: The kth prime number as description.

     */

    long long kthPrimeNumber(int k) {

        // write your code here

        vector<long long> v;

        v.push_back(1LL);

        int i = 0;

        int j = 0;

        int p = 0;

        int cnt = 0;

        while(cnt<k)

        {

            long long cur = min(min(v[i]*3,v[j]*5),v[p]*7);

            v.push_back(cur);

            while(v[i]*3<=cur)i++;

            while(v[j]*5<=cur)j++;

            while(v[p]*7<=cur)p++;

            cnt++;

        }

        return v.back();

    }

};