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Reverse Pairs

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Description

https://leetcode.com/problems/reverse-pairs/description/

Solution

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class Solution {
public:
    int reversePairs(vector<int>& nums) {
        int size = nums.size();
        if (size == 0 || size == 1) return 0;
        int ret = 0;
        findReversePair(nums, 0, size - 1, ret);
        // for (int i = 0; i < size; i++) cout << nums[i] << endl;
        return ret;
    }
    
    void findReversePair(vector<int>& nums, int left, int right, int& ret) {
        // cout << left << " " << right << endl;
        if (left >= right) return;
        
        int mid = left + (right - left) / 2;
        findReversePair(nums, left, mid, ret);
        findReversePair(nums, mid + 1, right, ret);
        
        //Check the number and sort the array
        int l = left;
        int r = mid + 1;
        while (l <= mid && r <= right) {
            while(l <= mid && nums[l]/2.0 <= nums[r]) {++l; ret += (r - mid - 1);}
            while(r <= right && nums[l]/2.0 > nums[r]) ++r;
        }
        while (l <= mid) {++l; ret += (r - mid - 1);}
        sort(nums.begin()+left, nums.begin()+right+1);
    }
};