luogu P1908 逆序对

二次联通门 : luogu P1908 逆序对

 

 

/*
    luogu P1908 逆序对
    
    权值线段树 + 离散化 + 指针版线段树。。。
    
    把所有数离散化后将其作为下标建空树 
     
    
    对于每次插入的数字x，
    查找x+1到max区间的数字存在的个数，
    并将这个个数加入答案，
    然后插入这个数字
    
    最后输出答案 
    
    %%Menci的线段树。。。6到一定境界了。。。 
*/
#include <algorithm>
#include <cstdio>


#define Max 40080


void read (int &now)
{
    now = 0;
    register char word = getchar ();
    while (word < '0' || word > '9')
        word = getchar ();    
    while (word >= '0' && word <= '9')
    {
        now = now * 10 + word - '0';
        word = getchar ();
    }
}

inline int min (int a, int b)
{
    return a < b ? a : b;
}

inline int max (int a, int b)
{
    return a > b ? a : b;
}

struct Segment_Tree_Date
{
    Segment_Tree_Date *Left, *Right;    
    
    Segment_Tree_Date ()
    {
        Left = NULL;
        Right = NULL;
        key = 0;
        
    }
    
    int l, r;    

    int key;
    int Mid;
};

Segment_Tree_Date *Root;

class Segment_Tree_Type
{
    public :
        
        
        void Build (Segment_Tree_Date *&now, int l, int r)
        {
            now = new Segment_Tree_Date;
            now->l = l;
            now->r = r;
            if (l == r)
                return ;
            now->Mid = l + r >> 1;
            Build (now->Left, l, now->Mid);
            Build (now->Right, now->Mid + 1, r);
        }
        
        void Change_Single (Segment_Tree_Date *&now, int pos)
        {
            if (now->l == now->r)
            {
                now->key++;
                return ;
            }
            if (pos <= now->Mid)
                Change_Single (now->Left, pos);
            else if (pos > now->Mid)
                Change_Single (now->Right, pos);
            now->key = now->Left->key + now->Right->key;
        }
        
        int Query (Segment_Tree_Date *&now, int l, int r)
        {
            if (l <= now->l && r >= now->r)
                return now->key;
            int res = 0;
            if (l <= now->Mid)
                res += Query (now->Left, l, min (now->Mid, r));
            if (r > now->Mid)
                res += Query (now->Right, max (l, now->Mid + 1), r);
            return res;
        }
}
Tree;

int N, M;

int date[Max];
int rank_number[Max];


int main (int argc, char *argv[])
{
    read (N);
    
    for (int i = 1; i <= N; i++)
    {
        read (date[i]);
        rank_number[i] = date[i];
    }
    std :: sort (date + 1, date + 1 + N);
    int Size = std :: unique (date + 1, date + 1 + N) - date - 1;
    int Answer = 0;
    Tree.Build (Root, 1, Size);
    for (int i = 1; i <= N; i++)
    {
        rank_number[i] = std :: lower_bound (date + 1, date + 1 + Size, rank_number[i]) - date;
        if (rank_number[i] < Size)
            Answer += Tree.Query (Root, rank_number[i] + 1, Size);
        Tree.Change_Single (Root, rank_number[i]);
    }
    printf ("%d", Answer);
    return 0;
}