codeforces D. Pashmak and Parmida's problem

http://codeforces.com/contest/459/problem/D

题意：给你n个数，然后统计多少组（i,j）使得f(1,i,ai)>f(j,n,aj);

思路：先从左往右统计f(1,i,ai)记录在b[i],然后从右往左统计f(j,n,aj)记录在c[i],然后利用树状数组统计个数即可。

 1 #include <cstdio>

 2 #include <iostream>

 3 #include <cstring>

 4 #include <cmath>

 5 #include <queue>

 6 #include <map>

 7 #include <algorithm>

 8 #define maxn 1000010

 9 #define ll long long

10 using namespace std;

11 const int inf=1<<30;

12 

13 int n,m;

14 ll a[maxn];

15 ll b[maxn];

16 ll c[maxn];

17 ll y[maxn];

18 ll xx[maxn];

19 ll vis1[maxn],vis2[maxn];

20 

21 int lowbit(int x)

22 {

23     return x&(-x);

24 }

25 

26 void Insert(int x,ll val)

27 {

28     while(x<maxn)

29     {

30         y[x]+=val;

31         x+=lowbit(x);

32     }

33 }

34 

35 ll Getsum(int x)

36 {

37     ll ans=0;

38     while(x>0)

39     {

40         ans+=y[x];

41         x-=lowbit(x);

42     }

43     return ans;

44 }

45 

46 int main()

47 {

48     scanf("%d",&n);

49     for(int i=1; i<=n; i++)

50     {

51         scanf("%lld",&a[i]);

52         xx[i]=a[i];

53     }

54     sort(xx+1,xx+1+n);

55     int cnt=1;

56     for(int i=2; i<=n; i++)

57     {

58         if(xx[i-1]!=xx[i])

59         {

60             xx[++cnt]=xx[i];

61         }

62     }

63     for(int i=1; i<=n; i++)

64     {

65         int pos=lower_bound(xx+1,xx+cnt+1,a[i])-xx;

66         vis1[pos]++;

67         b[i]=vis1[pos];

68     }

69     for(int i=n; i>=1; i--)

70     {

71         int pos=lower_bound(xx+1,xx+cnt+1,a[i])-xx;

72         vis2[pos]++;

73         c[i]=vis2[pos];

74     }

75     ll ans=0;

76     for(int i=n; i>=1; i--)

77     {

78         ans+=Getsum(b[i]-1);

79         Insert(c[i],1);

80     }

81     printf("%I64d\n",ans);

82     return 0;

83 }

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