2的幂次

package algorithm;

/**
 * Created by moi on 2017/10/18.
 */
public class PowerOf2 {

    public static int powOf2(int n){
        if(n==0){
            return 1;
        }
        if(n>31){
            throw new RuntimeException("n>31 is too big.");
        }
        int base = 2;
        while(n>1){
            base = base << 1;
            System.out.println(base);
            n--;
        }
        return base;
    }

    public static boolean checkPowerOf2(int n) {
        // write your code here
        boolean flag = true ;

        if ( n < 1 ){
            return false;
        }

        while( n>= 2 ){
            // 1的二进制是 最低位为1，其余各位为0，所以这里 n & 1 即n的最低位和 1的最低位1 进行与，如果是偶数，结果就是0，奇数就是1.
            if ( (n & 1) == 1 ){
                flag = false ;
                break;
            }
            n = n >> 1 ;
        }
        return flag ;
    }

    public static void main(String[] args) {
        PowerOf2.powOf2(3);
    }
}