其实就是丑数
class Solution {
public int getKthMagicNumber(int k) {
int[] res = new int[k];
int a = 0, b = 0, c = 0;
for (int i = 0; i < k ; i++) {
if (i == 0) {
res[i] = 1;
continue;
}
int n3 = res[a] * 3;
int n5 = res[b] * 5;
int n7 = res[c] * 7;
res[i] = Math.min(Math.min(n3, n5), n7);
if (res[i] == n3) a++;
if (res[i] == n5) b++;
if (res[i] == n7) c++;
}
return res[k-1];
}
}