<p>Given an integer array <code>nums</code>, return<em> an integer array </em><code>counts</code><em> where </em><code>counts[i]</code><em> is the number of smaller elements to the right of </em><code>nums[i]</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [5,2,6,1]
<strong>Output:</strong> [2,1,1,0]
<strong>Explanation:</strong>
To the right of 5 there are <b>2</b> smaller elements (2 and 1).
To the right of 2 there is only <b>1</b> smaller element (1).
To the right of 6 there is <b>1</b> smaller element (1).
To the right of 1 there is <b>0</b> smaller element.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [-1]
<strong>Output:</strong> [0]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> nums = [-1,-1]
<strong>Output:</strong> [0,0]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>-10<sup>4</sup> &lt;= nums[i] &lt;= 10<sup>4</sup></code></li>
</ul>

class Solution {
public:

    vector<pair<int,int>> insert(vector<pair<int,int>> &nums, int i, int j, vector<int> &ans){

        if(i==j){
            return vector<pair<int,int>> {{nums[i].first, nums[i].second}};
        }

        int mid = (i+j)/2;
        vector<pair<int,int>> a;
        vector<pair<int,int>> b;
        a = insert(nums, i, mid, ans);
        b = insert(nums, mid+1, j, ans);
        vector<pair<int,int>> final(a.size() + b.size());
        int apos =0, bpos=0,curpos=0;
        while(apos<a.size() && bpos<b.size()){
            if(a[apos] < b[bpos]){
                final[curpos] = a[apos];
                final[curpos] = a[apos];
                ans[final[curpos].second] += bpos;
                ++apos;
            }
            else{
                final[curpos] = b[bpos];
                final[curpos] = b[bpos];
                // ans[final[curpos].second] += apos;
                ++bpos;
            }
            ++curpos;
        }
        while(apos<a.size()){
            final[curpos] = a[apos];
            final[curpos] = a[apos];
            ans[final[curpos].second] += bpos;
            ++apos;
            ++curpos;
        }
        while(bpos<b.size()){
            final[curpos] = b[bpos];
            final[curpos] = b[bpos];
            // ans[final[curpos].second] += apos;
            ++bpos;
            ++curpos;
        }
        return final;
    }

    vector<int> countSmaller(vector<int>& nums) {
        int n = nums.size();
        vector<pair<int,int>> numss;
        vector<int> ans(n,0);
        for(int i=0; i<n; ++i)
            numss.push_back({nums[i],i});
        insert(numss, 0, n-1, ans);
        return ans;
    }
};
316593