第k个素因子只有3 5 7 的数

int findKth(int k) {
        // write code here
        if (k <= 0) return -1;

    vector<int> num;
    num.push_back(1);
    int p3 = 0, p5 = 0, p7 =0;
    while (num.size()<k + 1){

        int minnum = min((num[p3]) * 3, min((num[p5]) * 5, (num[p7]) * 7));
        num.push_back(minnum);
        while ((num[p3]) * 3 <= minnum)
            ++p3;
        while ((num[p5]) * 5 <= minnum)
            ++p5;
        while ((num[p7]) * 7 <= minnum)
            ++p7;
    }
    return num.back();
        
    }