二分查找算法

 package alg;import java.util.Arrays;/* * 算法分析: * 每执行一次search算法,查找的范围就小一半 * 假定有n个元素 * 当n=1时,查找一个元素需要常量时间T(n)=O(1); * 当n>1时,T(n)=T(n/2)+O(1); * T(n)=T(n/2)+O(1) *     =T(n/4)+2O(1) *     =T(n/2^3)+3O(1) *     =..... *     =T(n/2^n)+nO(1) *     令n=2^x *     则x=logn *     所以T(n)=T(1)+logn=O(1)+O(logn) 故其空间复杂性为O(logn) * */public class Select{ static int[] a={11,15,18,25,26,28,37}; public static int search(int a[],int x,int low,int high) {  int middle=(low+high)/2;  if(x==a[middle])   return middle;  else if(x>a[middle])  {    return search(a,x,middle+1,high);  }  else if(x<a[middle])  {   return search(a,x,low,middle-1);  }  return -1; }      public static void main(String[] args)     {      int x=search(a,26,0,a.length);      System.out.println(x);     }}