各种类型的二分模板

1、查找最后一个小于或者等于key的下标，若不存在，则返回-1 ==> 同lower_bound(arr+1, arr+1+n, key)

int BinSearch(int a[], int n, int key)
{
    int m, l = 0, r = n - 1;//闭区间[0, n - 1]
    while (l <= r)
    {
        m = (l+r)/2;
        if (a[m] <= key) l = m + 1;
        else if(a[m] > key) r = m - 1;
    }
    if(l == 0) return -1;//not find
    return l-1;
}

2、查找第一个大于或者等于key的下标，若不存在，则返回-1

int BinSearch(int a[], int n, int key)
{
    int m, l = 0, r = n - 1;//闭区间[0, n - 1]
    while (l <= r)
    {
        m = (l+r)/2;
        if (a[m] < key) l = m + 1;
        else if(a[m] >= key) r = m - 1;
    }
    if(r == n-1) return -1;//not find
    return r+1;
}

3、查找第一个与key相等的下标，若不存在，则返回-1

int BinSearch(int a[], int n, int key)
{
    int m, l = 0, r = n - 1;//闭区间[0, n - 1]
    while (l < r)
    {
        m = l + ((r - l) >> 1);//向下取整
        if (a[m] < key) l = m + 1;
        else r = m;
    }
    if (a[r] == key) return r;
    return -1;
}

4、查找最后一个与key相等的小标，若不存在，则返回-1

int BinSearch(int a[], int n, int key)
{
    int m, l = 0, r = n - 1;//闭区间[0, n - 1]
    while (l < r)
    {
        m = l + ((r + 1 - l) >> 1);//向上取整
        if (a[m] <= key) l = m;
        else r = m - 1;
    }
    if (a[l] == key) return l;
    return -1;
}

5、查找第一个大于key的下标，若不存在，则返回-1 ==> 同upper_bound(arr+1, arr+n+1, key)

int BinSearch(int a[], int n, int key)
{
    int m, l = 0, r = n - 1;//闭区间[0, n - 1]
    while (l < r)
    {
        m = l + ((r - l) >> 1);//向下取整
        if (a[m] <= key) l = m + 1;
        else r = m;
    }
    if (a[r] > key) return r;
    return -1;
}

6、查找最后一个小于key的下标，若不存在，则返回-1

int BinSearch(int a[], int n, int key)
{
    int m, l = 0, r = n - 1;//闭区间[0, n - 1]
    while (l < r)
    {
        m = l + ((r + 1 - l) >> 1);//向上取整
        if (a[m] < key) l = m;
        else r = m - 1;
    }
    if (a[l] < key) return l;
    return -1;
}