hdu 1394 逆序对(nlgn+o(n) )
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Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 20404 Accepted Submission(s): 12231
Problem Description
The inversion number of a given number sequence a1, a2, …, an is the number of pairs (ai, aj) that satisfy i < j and ai > aj.
For a given sequence of numbers a1, a2, …, an, if we move the first m >= 0 numbers to the end of the seqence, we will obtain another sequence. There are totally n such sequences as the following:
a1, a2, …, an-1, an (where m = 0 - the initial seqence)
a2, a3, …, an, a1 (where m = 1)
a3, a4, …, an, a1, a2 (where m = 2)
…
an, a1, a2, …, an-1 (where m = n-1)
You are asked to write a program to find the minimum inversion number out of the above sequences.
Input
The input consists of a number of test cases. Each case consists of two lines: the first line contains a positive integer n (n <= 5000); the next line contains a permutation of the n integers from 0 to n-1.
Output
For each case, output the minimum inversion number on a single line.
Sample Input
10
1 3 6 9 0 8 5 7 4 2
Sample Output
16
思路:先求出原序列的逆序对和,在递推求变换位置的和,取最小值,用线段树叶子维护0~n-1的值
#include <cstdio>#include <iostream>#include <algorithm>using namespace std;#define lson l,m,rt<<1 #define rson m+1,r,rt<<1|1const int maxn=5000+100;int sum[maxn<<2];void pushup(int rt) { sum[rt]=sum[rt<<1]+sum[rt<<1|1];}void build(int l,int r,int rt) { sum[rt]=0; if(l==r) return ; int m=(l+r)>>1; build(lson); build(rson);//no need pushup all 0}void update(int p,int l,int r,int rt) { if(l==r) { sum[rt]=1; return ; } int m=(l+r)>>1; if(p<=m) update(p,lson); else update(p,rson); pushup(rt);}int query(int L,int R,int l,int r,int rt) { if(L<=l&&r<=R) { return sum[rt]; } int m=(l+r)>>1; int ans=0; if(L<=m) ans+=query(L,R,lson); if(R>m) ans+=query(L,R,rson); return ans; }int a[maxn];int main() {// freopen("input.txt","r",stdin); int n; while(scanf("%d",&n)!=EOF) { int ans=0; build(0,n-1,1); for(int i=0;i<n;i++) { scanf("%d",&a[i]); ans+=query(a[i],n-1,0,n-1,1); update(a[i],0,n-1,1); } int ret=ans; for(int i=0;i<n;i++) { ans+=n-a[i]-a[i]-1; ret=min(ret,ans); } printf("%d\n",ret); } return 0;}
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