二分查找

 

//循环实现

int binsearch(int* a, int i, int j, int goal){
    if(a == NULL)
        return -1;
    while(i<=j){
        int mid = (i + j)/2;
        if(a[mid] == goal){
            return mid;
        }
        else if(a[mid] > goal){
            j = mid - 1;
        }
        else if(a[mid] < goal){
            i = mid + 1;
        }
    }
    return -1;
}

递归实现

int binsearch(int* a, int i, int j, int goal){
    if(a == NULL)
        return -1;
    if(i > j)
        return -1;
    int mid = (i + j)/2;
    if(a[mid] == goal){
        return mid;
    }
    else if(a[mid] > goal){
        return binsearch(a,i,mid-1,goal);
    }
    else if(a[mid] < goal){
        return binsearch(a,mid+1,j,goal);
    }
}