点击打开hdu 2838

思路: 树状数组

分析:

1 题目和求逆序数那题很像，裸题

代码:

/************************************************
 * By: chenguolin                               * 
 * Date: 2013-08-21                             *
 * Address: http://blog.csdn.net/chenguolinblog *
 ***********************************************/
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;

typedef __int64 int64; 
const int MAXN = 100010;

int n , num[MAXN];
int64 treeNum[MAXN];
int64 treeSum[MAXN];

int lowbit(int x){
    return x&(-x);
}

int64 getSum(int x , int64 *arr){
    int64 sum = 0;
    while(x){
        sum += arr[x];
        x -= lowbit(x);
    }
    return sum;
}

void add(int x , int val , int64 *arr){
    while(x < MAXN){
        arr[x] += val;
        x += lowbit(x);
    }
}

int64 solve(){
    memset(treeNum , 0 , sizeof(treeNum)); 
    memset(treeSum , 0 , sizeof(treeSum)); 
    int64 ans = 0;
    int64 total = 0;
    for(int i = 1 ; i <= n ; i++){
        if(i > 1){
            ans += total-getSum(num[i]-1 , treeSum);
            ans += (i-1-getSum(num[i]-1 , treeNum))*num[i];
        }
        total += num[i];
        add(num[i] , 1 , treeNum);
        add(num[i] , num[i] , treeSum);
    }
    return ans;
}

int main(){
    while(scanf("%d" , &n) != EOF){
        for(int i = 1 ; i <= n ; i++)
            scanf("%d" , &num[i]);
        printf("%I64dd\n" , solve());
    }
    return 0;
}