每日一记--2014.9.13

今天贴三个小程序，程序很小，但是希望这些小东西都能记在心里

1.求多项式

package 多项式;

public class Polynomial {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] xishu = new int[]{2,1,0,8,4};
        System.out.println(poly(xishu,3));
        
    }
    private static long poly(int[] xishu,int x){
        long po = 0L;
        for(int i = xishu.length-1;i>=0;i--){
            po=po*x+xishu[i];//注意系数是从an开始递减到a1的
        }    
        return po;
    }

}

2.判断一个数是否为素数

package 判断素数;

public class PrimeNumber {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println(isPrime(71));
    }

    private static boolean isPrime(int a) {
        if(a<2){
            return false;
        }
        else{
            for(int i=2;i<Math.sqrt(a);i++){
                if(a%i==0)
                    return false;
            }
        }
        return true;
    }

}

3. 二分查找 

   时间复杂度为o(logN)

package binarySearch;
//折半查找必须针对已排序的数列
public class BinarySearch {

    static int NOT_FOUND = -1;
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] aa = new int[]{2,3,7,89,100,256,445,789};
        System.out.println(binarySearch(aa,100));
    }
    private static int binarySearch(int[] sortedArray ,int find){
        int low =0;
        int high = sortedArray.length-1;
        while(low<=high){
            int mid = (low+high)/2;
            if(sortedArray[mid]<find){
                low=mid+1;
            }
            else if(sortedArray[mid]>find){
                high=mid-1;
            }
            else
                return mid;
        }
        return NOT_FOUND;
    }

}