HDU4911 Inversion 解题报告

题意：求逆序对

解题思路：1)树状数组 + 离散化

解题代码：

// File Name: a.cpp
// Author: darkdream
// Created Time: 2014年08月05日 星期二 12时05分09秒

#include<vector>
#include<list>
#include<map>
#include<set>
#include<deque>
#include<stack>
#include<bitset>
#include<algorithm>
#include<functional>
#include<numeric>
#include<utility>
#include<sstream>
#include<iostream>
#include<iomanip>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<ctime>
#define LL long long

using namespace std;
int n , k ; 
LL sum = 0 ; 
int lowbit(int x)
{
    return x&(-x);
}
struct node{
    int num, s,rs ; 
}a[100005];
int num[100005];
int getsum(int x){
    int sum = 0 ;
    while(x >= 1)
    {
        sum += num[x];
        x -= lowbit(x);
    }
    return sum ; 
}
void update(int x)
{
    while(x <= n )
    {
        num[x] ++ ; 
        x += lowbit(x);
    }
    return; 
}
int cmp(node x ,node y )
{
    return x.num < y.num;
}
int cmp1(node x, node y)
{
    return x.s < y.s; 
}
int main(){
    while(scanf("%d %d",&n,&k)!= EOF)
    {
        memset(num,0,sizeof(num));
        for(int i =1;i <= n;i ++)
        {
            scanf("%d",&a[i].num);
            a[i].s = i  ;
        }
        sort(a+1,a+1+n,cmp);
        int t = 1; 
        a[1].rs = 1; 
        for(int i = 2;i <= n;i ++)
        {
            if(a[i].num == a[i-1].num) 
                a[i].rs = t ;
            else {
                t++;
                a[i].rs = t; 
            }
        }
        sort(a+1,a+1+n,cmp1);
        sum = 0 ;
        for(int i = 1;i <= n;i ++)
        {
            sum += (getsum(n)- getsum(a[i].rs));
            update(a[i].rs);
        }
        if(sum > k)
          cout << sum-k << endl;
        else printf("0
");
    }
    return 0;
}

2）归并排序