<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:
void merge(int l,int h,vector<pair<int,int>>&v,vector<int>&ans){
    int mid=(l+h)/2;
    vector<pair<int,int>>a,b;
    for(int i=l;i<=h;i++){
        if(i<=mid){
        a.push_back(v[i]);
        }
        else{
            b.push_back(v[i]);
        }
    }
    int index=l;
    int p1=0,p2=0;
    while(p1<(int)a.size() && p2<(int)b.size()){
        if(a[p1].first<=b[p2].first){
            ans[a[p1].second]+=p2;
              v[index]=a[p1];
              index++;
              p1++;
        }
        else if(a[p1].first>b[p2].first){
            v[index]=b[p2];
            index++;
            p2++;
        }
    }
    if(p1==(int)a.size()){
        for(int i=p2;i<(int)b.size();i++){
            v[index]=b[i];
            index++;
        }
    }
    else{
        for(int i=p1;i<(int)a.size();i++){
            ans[a[i].second]+=p2;
            v[index]=a[i];
            index++;
        }
    }   
}
  void solve(int l,int h,vector<pair<int,int>>&v,vector<int>&ans){
    if(l==h){
        return ;
    }
    int mid=(l+h)/2;
    solve(l,mid,v,ans);
    solve(mid+1,h,v,ans);
    merge(l,h,v,ans);
  }
    vector<int> countSmaller(vector<int>& nums) {
        if((int)nums.size()==1){
            return {0};
        }
        else{
             vector<pair<int,int>>v;
             for(int i=0;i<(int)nums.size();i++){
                   v.push_back({nums[i],i});
             }
             vector<int>ans((int)nums.size());
             solve(0,(int)nums.size()-1,v,ans);
             for(auto i:v){
                cout<<i.first<<" "<<i.second<<"\n";
             }
             return ans;
        }
    }
};
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