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CountInversions.cpp
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192 lines (164 loc) · 3.68 KB
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#include <bits/stdc++.h>
using namespace std;
void invCount(vector<int> &count, vector<pair<int, int>> &v, int low, int high)
{
int mid=low+(high-low)/2, i=low, k=0, j=mid+1;
vector<pair<int, int>> temp(high-low+1);
while(i<=mid && j<=high)
{
if(v[i].first>v[j].first)
{
count[v[i].second] += high-j+1;
temp[k++]=v[i++];
}
else temp[k++]=v[j++];
}
while(i<=mid)
temp[k++]=v[i++];
while(j<=high)
temp[k++]=v[j++];
for(int x=low; x<=high; x++)
{
v[x]=temp[x-low];
}
}
void mergeSort(vector<int> &count, vector<pair<int, int>> &v, int low, int high)
{
if(low>=high) return;
int mid=low+(high-low)/2;
mergeSort(count, v, low, mid);
mergeSort(count, v, mid+1, high);
invCount(count, v, low, high);
}
vector<int> countSmaller(vector<int>& nums) {
int n=nums.size();
vector<pair<int, int>> v;
vector<int> count(n,0);
for(int i=0; i<n; i++)
v.push_back(make_pair(nums[i], i));
mergeSort(count, v, 0, n-1);
return count;
}
int main()
{
//cout<<"Hello World";
vector<int>v={5,2,6,1};
vector<int>res=countSmaller(v);
for(auto it:res)
cout<<it<<" ";
return 0;
}
/*
#include <bits/stdc++.h>
using namespace std;
int _mergeSort(int arr[], int temp[], int left, int right);
int merge(int arr[], int temp[], int left, int mid, int right);
int mergeSort(int arr[], int array_size)
{
int temp[array_size];
return _mergeSort(arr, temp, 0, array_size - 1);
}
int _mergeSort(int arr[], int temp[], int left, int right)
{
int mid, inv_count = 0;
if (right > left) {
mid = (right + left) / 2;
inv_count += _mergeSort(arr, temp, left, mid);
inv_count += _mergeSort(arr, temp, mid + 1, right);
inv_count += merge(arr, temp, left, mid + 1, right);
}
return inv_count;
}
int merge(int arr[], int temp[], int left, int mid, int right)
{
int i, j, k;
int inv_count = 0;
i = left;
j = mid;
k = left;
while ((i <= mid - 1) && (j <= right)) {
if (arr[i] <= arr[j]) {
temp[k++] = arr[i++];
}
else {
temp[k++] = arr[j++];
inv_count = inv_count + (mid - i);
}
}
while (i <= mid - 1)
temp[k++] = arr[i++];
while (j <= right)
temp[k++] = arr[j++];
vector<int>v1;
for (i = left; i <= right; i++)
{
arr[i] = temp[i];
v1.push_back(temp[i]);
}
return inv_count;
}
// Driver code
int main()
{
int arr[] = { 2,4,1,3,5 };
int n = sizeof(arr) / sizeof(arr[0]);
int ans = mergeSort(arr, n);
cout << " Number of inversions are " << ans;
return 0;
}
*/
/*
#include<bits/stdc++.h>
using namespace std;
int merge(int arr[],int temp[],int left,int mid,int right)
{
int i,j,k,inv_cnt=0;
i=left;
j=mid;
k=left;
while((i<=mid-1) && (j<=right))
{
if(arr[i]<=arr[j])
temp[k++]=arr[i++];
else
{
temp[k++]=arr[j++];
inv_cnt+=(mid-i);
}
}
while(i<=mid)
temp[k++]=arr[i++];
while(j<=right)
temp[k++]=arr[j++];
for(int i=left;i<=right;i++)
{
arr[i]=temp[i];
}
return inv_cnt;
}
int mergeSortSpliting(int arr[],int temp[],int left,int right)
{
int mid,inv_cnt=0;
if(right>left)
{
mid=(right+left)/2;
inv_cnt+=mergeSortSpliting(arr,temp,left,mid);
inv_cnt+=mergeSortSpliting(arr,temp,mid+1,right);
inv_cnt+=merge(arr,temp,left,mid+1,right);
}
return inv_cnt;
}
int _mergeSortAppr(int arr[],int n)
{
int temp[n];
return mergeSortSpliting(arr,temp,0,n-1);
}
int main()
{
int a[]={2,4,1,3,5};
int n=sizeof(a)/sizeof(a[0]);
int res=_mergeSortAppr(a,n);
cout<<res;
return 0;
}
*/