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

 1 package 多项式;
 2
 3 public class Polynomial {
 4
 5     public static void main(String[] args) {
 6         // TODO Auto-generated method stub
 7         int[] xishu = new int[]{2,1,0,8,4};
 8         System.out.println(poly(xishu,3));
 9
10     }
11     private static long poly(int[] xishu,int x){
12         long po = 0L;
13         for(int i = xishu.length-1;i>=0;i--){
14             po=po*x+xishu[i];//注意系数是从an开始递减到a1的
15         }
16         return po;
17     }
18
19 }

2.判断一个数是否为素数

 1 package 判断素数;
 2
 3 public class PrimeNumber {
 4
 5     public static void main(String[] args) {
 6         // TODO Auto-generated method stub
 7         System.out.println(isPrime(71));
 8     }
 9
10     private static boolean isPrime(int a) {
11         if(a<2){
12             return false;
13         }
14         else{
15             for(int i=2;i<Math.sqrt(a);i++){
16                 if(a%i==0)
17                     return false;
18             }
19         }
20         return true;
21     }
22
23 }

3. 二分查找

时间复杂度为o(logN)

 1 package binarySearch;
 2 //折半查找必须针对已排序的数列
 3 public class BinarySearch {
 4
 5     static int NOT_FOUND = -1;
 6     public static void main(String[] args) {
 7         // TODO Auto-generated method stub
 8         int[] aa = new int[]{2,3,7,89,100,256,445,789};
 9         System.out.println(binarySearch(aa,100));
10     }
11     private static int binarySearch(int[] sortedArray ,int find){
12         int low =0;
13         int high = sortedArray.length-1;
14         while(low<=high){
15             int mid = (low+high)/2;
16             if(sortedArray[mid]<find){
17                 low=mid+1;
18             }
19             else if(sortedArray[mid]>find){
20                 high=mid-1;
21             }
22             else
23                 return mid;
24         }
25         return NOT_FOUND;
26     }
27
28 }