二分查找

是在已排好序的n个元素数组arr [0：n-1] 中找某一特定元素goal。

不断的将数组对半分割，将中间元素与目标元素相比较。

注意：①元素是排好序的。

            ②偶数元素对半分割舍弃小数，是前一个元素。

#include <iostream>
#include <algorithm>  //sort函数对应的头文件
#define MAX 101

using namespace std;

int Binary_Search(int a[], int len, int goal)
{
    int low = 0;
    int high = len-1;
    while(low <= high)
    {
        int mid = (low + high)/2;
        if( a[mid] == goal)
            return mid;
        else if(a[mid] < goal)
            low = mid + 1;
        else if(a[mid] > goal)
            high = mid - 1;
    }
    return -1;
}
int main()
{
    int arr[MAX];
    int len, x, goal;
    cout<<"Please input the number of all elements: ";
    cin>>len;
    cout<<"\nPlease input the elements: \n";
    for(int i = 0; i < len; i++)
    {
        cin>>x;
        arr[i] = x;
    }
    sort(arr,arr+len); //排序函数
    cout<<"\nPlease input the objective element: \n";
    cin>>goal;
    int answer = Binary_Search(arr, len, goal);
    if(answer == -1)
        cout<<"\nDidn't find.\n";
    else
        cout<<"the subscript of the objective element is: "<<answer<<endl;
    return 0;
}