二分查找

#include <iostream>
using namespace std;
const int N=5;
int a[N]= {1,3,5,7,9};
int binary_search(int x);

int main()
{
    int result=binary_search(4);
    if (result==-1)
        cout << "not found!" << endl;
    else
        cout << result << endl;
    return 0;
}

int binary_search(int x)
{
    int left,right,mid,guess;
    left=0;
    right=N-1;
    while(left<=right)//要允许=否则最后一个数找不到
    {
        mid=left+(right-left)/2;//避免数据较大时越界
        guess=a[mid];
        if (guess==x)
            return mid;
        if (guess>x)
            right=mid-1;
        else
            left=mid+1;
    }
    return -1;
}