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18_ReversePairs.cpp
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18_ReversePairs.cpp
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#include<iostream>
#include<bits/stdc++.h>
using namespace std;
class Solution {
public:
void merge(vector<int>& nums, int low, int mid, int high, int& reversePairsCount){
int j = mid+1;
for(int i=low; i<=mid; i++){
while(j<=high && nums[i] > 2*(long long)nums[j]){
j++;
}
reversePairsCount += j-(mid+1);
}
int size = high-low+1;
vector<int> temp(size, 0);
int left = low, right = mid+1, k=0;
while(left<=mid && right<=high){
if(nums[left] < nums[right]){
temp[k++] = nums[left++];
}
else{
temp[k++] = nums[right++];
}
}
while(left<=mid){
temp[k++] = nums[left++];
}
while(right<=high){
temp[k++] = nums[right++];
}
int m=0;
for(int i=low; i<=high; i++){
nums[i] = temp[m++];
}
}
void mergeSort(vector<int>& nums, int low, int high, int& reversePairsCount){
if(low >= high){
return;
}
int mid = (low + high) >> 1;
mergeSort(nums, low, mid, reversePairsCount);
mergeSort(nums, mid+1, high, reversePairsCount);
merge(nums, low, mid, high, reversePairsCount);
}
int reversePairs(vector<int>& nums) {
int reversePairsCount = 0;
mergeSort(nums, 0, nums.size()-1, reversePairsCount);
return reversePairsCount;
}
};
int main()
{
return 0;
}