逆序数c++代码_C++程序设计

逆序数（C/C++）

1. O(n^2)

#include <stdio.h>

int ReversedCount(int A[],int n){ 
   
    int Sum = 0;
    for (int i = 1; i <= n; i++)
    { 
   
        for (int j = i; j <= n; j++)
        { 
   
            if (A[i]>A[j])
            { 
   
                Sum += 1;
            }
        }
    }
    return Sum;
}
int main(int argc, char const *argv[])
{ 
   
    int n;
    int a[100001];
    scanf(" %d", &n);
    getchar();
    for (int i = 1; i <= n; i++)
    { 
   
        scanf("%d", &a[i]);
    }

    int cnt=ReversedCount(a,n);
    printf("%d",cnt);

    return 0;
}

2. 利用归并（分治）

#include <iostream>
using namespace std;


int B[100001];
long s=0;
void Merge(int A[],int low,int mid,int high){
    int i,j,k;
    for (k = low; k <= high; k++)
    {
        B[k] = A[k];
    }
    for ( i = low,j=mid+1,k=low; i <= mid && j<=high; k++)
    {
        if (B[i]<=B[j])
        {
            A[k]=B[i++];
        }
        else{
            A[k]=B[j++];
            s+=(mid-i+1);
        }
        
    }
    while (i<=mid)
    {
        A[k++]=B[i++];
    }
    while (j<=high)
    {
        A[k++]=B[j++];
    }
    
}

void MerfeSort(int A[],int low,int high){
    if(low<high){
        int mid=low+(high-low)/2;
        MerfeSort(A,low,mid);
        MerfeSort(A,mid+1,high);
        Merge(A,low,mid,high);
    }
    return;
}
void PrintFun(int A[],int n){
    for (int i = 0; i < n; i++)
    {
        printf("%d  ",A[i]);
    }
}

int main(int argc, char const *argv[])
{
    int n;
    int A[100001];
    scanf(" %d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%d", &A[i]);
    }

    MerfeSort(A,0,n-1);
    // PrintFun(A,n);
    //printf("%d",s);
    cout << s << endl;
    return 0;
}

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