[2016-05-10][51nod][1019 逆序数]

• 时间:2016-05-10 15:33:59 星期二

• 题目编号:[2016-05-10][51nod][1019 逆序数]

• 题目大意:求一列数的逆序数

• 分析:

  • 分治
  • 把A平均分成B,C两列
  • 逆序对(I,j),要么I,j都属于B,要么i,j都属于C,要么I属于B,j属于C,

#include<stdio.h>
#include<string.h>
#include<vector>
using namespace std;
typedef long long ll;
vector<int> A;
ll merge_count(vector<int> & a){
        int n = a.size();
        if(n <= 1)      return 0;
        ll cnt = 0;
        vector<int> b(a.begin(),a.begin() + n / 2);
        vector<int> c(a.begin() + n / 2,a.end());
        cnt += merge_count(b);
        cnt += merge_count(c);
        int ai = 0,bi = 0,ci = 0;
        while(ai < n ){
                if(bi < b.size() && (ci == c.size() || b[bi] <= c[ci])){
                        a[ai++] = b[bi++];
                }else {
                        cnt += n / 2 - bi;
                        a[ai++] = c[ci++];
                }
        }
        return cnt;
}
int main(){
        int n,tmp;
        scanf("%d",&n);
        A.resize(n + 1);
        A.clear();
        for(int i = 0 ; i < n ; ++i){
                scanf("%d",&tmp);
                A.push_back(tmp);
        }
        printf("%lld\n",merge_count(A));
        return 0;
}

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