二分查找 θ(logn)

递归方法
int binarySearch1(int a[] , int low , int high , int findNum)
{   
int mid = ( low + high ) / 2;      
if (low > high)       
return -1;   
else   
{       
if (a[mid] > findNum)          
  return binarySearch1(a, low, mid - 1, findNum);       
else if (a[mid] < findNum)           
return binarySearch1(a, mid + 1, high, findNum);       
else           
return mid;   
}
}

非递归方法
int binarySearch2(int a[] , int low , int high , int findNum)
{   
while (low <= high)
    {
        int mid = ( low + high) / 2;   //此处一定要放在while里面
            if (a[mid] < findNum)           
low = mid + 1;       
else if (a[mid] > findNum)           
high = mid - 1;       
else           
return mid;   
}      
return -1;
}