Pow_a_b_efficiently

gistfile1.txt

package interview;

import org.junit.Test;

import java.util.concurrent.ConcurrentHashMap;

import static org.junit.Assert.assertEquals;

/**
 * efficiently  implement a^b
 * Created by zhuguowei on 5/16/17.
 */
public class Pow {
    private static final ConcurrentHashMap<String,Long> cache = new ConcurrentHashMap();
//    private static final HashMap<String,Long> cache = new HashMap();

    public long pow(long a, long b){
        System.out.printf("%d ^ %d%n",a,b);
        if(b == 1L){
            return a;
        }
        if( b == 2L){
            return a*a;
        }
        long l = b/2;
        long r = b - l;

        return cache.computeIfAbsent(a+","+l,k->pow(a,l)) * cache.computeIfAbsent(a+","+r,k->pow(a,r));
    }
    @Test
    public void testPow(){
        long result = pow(2, 30);
        assertEquals(Math.pow(2,30),result*1.0,0);
    }
    
        public long pow2(final long a, final long b){
//        return LongStream.range(0, b).parallel().reduce(1, (acc, x) -> a * acc);
        return Collections.nCopies((int)b,a).parallelStream().reduce(1L, (acc, x) -> acc * x);
    }
}