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：

 1 // File Name: a.cpp
 2 // Author: darkdream
 3 // Created Time: 2014年08月05日 星期二 12时05分09秒
 4 
 5 #include<vector>
 6 #include<list>
 7 #include<map>
 8 #include<set>
 9 #include<deque>
10 #include<stack>
11 #include<bitset>
12 #include<algorithm>
13 #include<functional>
14 #include<numeric>
15 #include<utility>
16 #include<sstream>
17 #include<iostream>
18 #include<iomanip>
19 #include<cstdio>
20 #include<cmath>
21 #include<cstdlib>
22 #include<cstring>
23 #include<ctime>
24 #define LL long long
25 
26 using namespace std;
27 int n , k ; 
28 LL sum = 0 ; 
29 int lowbit(int x)
30 {
31     return x&(-x);
32 }
33 struct node{
34     int num, s,rs ; 
35 }a[100005];
36 int num[100005];
37 int getsum(int x){
38     int sum = 0 ;
39     while(x >= 1)
40     {
41         sum += num[x];
42         x -= lowbit(x);
43     }
44     return sum ; 
45 }
46 void update(int x)
47 {
48     while(x <= n )
49     {
50         num[x] ++ ; 
51         x += lowbit(x);
52     }
53     return; 
54 }
55 int cmp(node x ,node y )
56 {
57     return x.num < y.num;
58 }
59 int cmp1(node x, node y)
60 {
61     return x.s < y.s; 
62 }
63 int main(){
64     while(scanf("%d %d",&n,&k)!= EOF)
65     {
66         memset(num,0,sizeof(num));
67         for(int i =1;i <= n;i ++)
68         {
69             scanf("%d",&a[i].num);
70             a[i].s = i  ;
71         }
72         sort(a+1,a+1+n,cmp);
73         int t = 1; 
74         a[1].rs = 1; 
75         for(int i = 2;i <= n;i ++)
76         {
77             if(a[i].num == a[i-1].num) 
78                 a[i].rs = t ;
79             else {
80                 t++;
81                 a[i].rs = t; 
82             }
83         }
84         sort(a+1,a+1+n,cmp1);
85         sum = 0 ;
86         for(int i = 1;i <= n;i ++)
87         {
88             sum += (getsum(n)- getsum(a[i].rs));
89             update(a[i].rs);
90         }
91         if(sum > k)
92           cout << sum-k << endl;
93         else printf("0\n");
94     }
95     return 0;
96 }