二分查找

该适用在有序的序列中查找

int binary_search(int arr[],int key,int n){
    int low=0,high=n-1;
    int mid;
    while(low<=high){
        mid=(low+high)/2;
        if(arr[mid]<key){
            low=mid+1;
        }else if(arr[mid]>key){
            high=mid-1;
        }else{
            return mid;
        }
    }
    return -1; //not found
}