hdu-1394Minimum Inversion Number(暴力解法或者线段树 求最少逆序对)
2016-08-26 11:32
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Minimum Inversion Number
Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)Total Submission(s): 18319 Accepted Submission(s): 11123
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
Author
CHEN, Gaoli
Source
ZOJ Monthly, January 2003
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暴力解法
#include<iostream> #include<string> #include<cstring> #include<cstdio> using namespace std; int a[5500]; int n; int main() { while(~scanf("%d",&n)) //总共最多有n-1种逆数对 { for(int i=0;i<n;i++) { scanf("%d",&a[i]); } int ans=0; for(int i=0;i<n;i++)//求最开始的逆数对有多少 { for(int j=i+1;j<n;j++) { if(a[i]>a[j]) ans++; } } int res=999999999; if(ans<res)res=ans; for(int i=0;i<n;i++) { ans=ans-a[i]+n-1-a[i];//如果将a[i]移置最后 那么逆数对将减少a[i]中 但是反之会增加n-1-a[i]种逆数对 if(ans<res)res=ans; } printf("%d\n",res); } return 0; } /* 10 1 3 6 9 0 8 5 7 4 2 16 */
线段树解法
#include<iostream> #include<string> #include<cstring> #include<cstdio> using namespace std; #define N 5555 #define lson l,m,rt<<1 #define rson m+1,r,rt<<1|1 int sum[N<<2]; void pushup(int rt) { sum[rt]=sum[rt<<1]+sum[rt<<1|1]; } void build(int l,int r,int rt)//初始化 { sum[rt]=0; if(l==r) { return; } int m=(l+r)>>1; build(lson); build(rson); pushup(rt); } void update(int p,int l,int r,int rt) { if(l==r) { sum[rt]++;//由于存入了p这个点 sum[rt]的数量要加一 在递归更新节点的sum值 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)//将L到R之间有多少点存在的数量 求出来 { if(L<=l&&R>=r) { return sum[rt]; } int m=(l+r)>>1; int res=0; if(L<=m)res+=query(L,R,lson); if(R>m)res+=query(L,R,rson); return res; } int x ; int main() { int n; while(~scanf("%d",&n)) { build(0,n-1,1); int sum=0; for(int i=0;i<n;i++) { scanf("%d",&x[i]); sum+=query(x[i],n-1,0,n-1,1);//逆序对 当插入x[i]时 查找x[i]到n-1的区间有多少之前已经存入的点 有多少个即说明增加了多少个逆序对 update(x[i],0,n-1,1);//将x[i]数据更新到线段树中 } int res=sum; for(int i=0;i<n;i++) { sum=sum-x[i]+n-1-x[i];////如果将a[i]移置最后 那么逆数对将减少a[i]中 但是反之会增加n-1-a[i]种逆数对 循环一遍 查找最小逆序对的数量 res=min(sum,res); } cout<<res<<endl; } return 0; } /* 10 1 3 6 9 0 8 5 7 4 2 16 */
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