<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>

#include <ext/pb_ds/assoc_container.hpp>
 
#include <functional> // for less
using namespace __gnu_pbds ;
#define ordered_set tree<int,null_type,less_equal<int>,rb_tree_tag,tree_order_statistics_node_update> 
class Solution {
public:
vector<int>ans ;
void f(vector<vector<int>>&nums , int l , int m , int h , vector<int>&num){
    int j = m+1 ;
    for(int i=l;i<=m;i++){
        while(j <= h && nums[i][0] > nums[j][0]){
            j++ ;
        }
        ans[nums[i][1]] += j-m-1 ;
    }
}
void merge( vector<vector<int>>&nums , int l , int h , vector<int>&num){
    if(l>=h) return ;
    int m =(l+h)/2 ;
    merge(nums , l , m , num) ;
    merge(nums , m+1 , h,num) ;
    f(nums , l, m, h,num) ;
    inplace_merge(nums.begin()+l,nums.begin()+m+1,nums.begin()+h+1 ) ;
}
    vector<int> countSmaller(vector<int>& num){
        
        int n = num.size() ;
        ans.resize(n) ;
       vector<vector<int>>nums(n) ;
      for(int i=0;i<n;i++){
        nums[i] = {num[i] , i} ;
      }
       
      merge(nums , 0 , n-1,num ) ;
       return ans ;
    }
};
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