二分搜索

#include<iostream>
using namespace std;

//前置条件：数组a已经排序，left和right是搜索区间的开始和结束位置，X为要查找的元素

//后置条件：若搜索成功，返回X在数组a中的索引，否则，返回-1
int BinarySerch(int *a,int left,int right,int X);
int main()
{
 int a[]={1,2,3,4,5,6,7,8,9,10};
 cout<<BinarySerch(a,0,9,5)<<endl;
 cout<<BinarySerch(a,-1,9,10)<<endl;
 cout<<BinarySerch(a,0,9,11)<<endl;
 system("PAUSE");
 return 0;
}
int BinarySerch(int *a,int left,int right,int X)
{
 if(a==NULL||left<0)return -1;
 int mid;
 while(left<=right)
 {
  mid=(left+right)>>1;
  if(a[mid]==X)return mid;
  else if(a[mid]<X) left=mid+1;
  else right=mid-1;
 }
 return -1;
}