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reverse_pairs.cpp
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reverse_pairs.cpp
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/*
Problem Link: https://leetcode.com/problems/reverse-pairs/
*/
class Solution {
public:
// similar to merge sort: count inversions
// helper function to count inversions
int merge(vector<int> &nums, int start, int mid, int end) {
int count = 0;
int n1 = mid - start + 1; // mid is included
int n2 = end - mid;
// fill the values
int left[n1], right[n2];
for(int i = 0; i < n1; i++)
left[i] = nums[start + i];
for(int i = 0; i < n2; i++)
right[i] = nums[mid + i + 1];
// count the required pairs
// traverse the left part of the array
int j = 0;
for(int i = 0; i < n1; i++) {
while(j < n2 && left[i] > 2LL * right[j])
j++;
count += j;
}
// 3 pointers
int i = 0, k = start;
j = 0;
while(i < n1 && j < n2) {
if(left[i] <= right[j])
nums[k++] = left[i++];
else
nums[k++] = right[j++];
}
// add the remaining elements
while(i < n1)
nums[k++] = left[i++];
while(j < n2)
nums[k++] = right[j++];
return count;
}
// recursive mergeSort function
int mergeSort(vector<int> &nums, int start, int end) {
// base case
if(start >= end)
return 0;
int mid = start + (end - start) / 2;
int count = mergeSort(nums, start, mid);
count += mergeSort(nums, mid + 1, end);
// cross inversions
count += merge(nums, start, mid, end);
return count;
}
int reversePairs(vector<int>& nums) {
return mergeSort(nums, 0, nums.size() - 1);
}
};