<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 left, int mid, int right, vector<pair<int, int>>& arr,vector<int>& count)
    {
        vector<pair<int, int>> temp(right - left + 1);
        
        int i = left;
        int j = mid + 1;
        int k = 0;
        
        while(i <= mid && j <= right)
        {
            if(arr[i].first <= arr[j].first)
            {
                temp[k++] = arr[j++]; 
            }
            else
            {
                count[arr[i].second] += (right - j + 1);
                
                temp[k++] = arr[i++];
            }
        }
        
        while(i <= mid)
        {
            temp[k++] = arr[i++];
        }
        
        while(j <= right)
        {
            temp[k++] = arr[j++];
        }
        
        for(int l = left; l <= right; l++)
        arr[l] = temp[l - left];
        
    }
                
    void mergeSort(int left, int right, vector<pair<int, int>>& arr, vector<int>& count)
    {
        if(left >= right)
        {
            return;
        }

        int mid = left + (right - left) / 2;
        
        mergeSort(left, mid, arr, count);
        mergeSort(mid + 1, right, arr, count);
        
        merge(left, mid, right, arr, count);
    }
    
	vector<int> countSmaller(vector<int>& nums) {
	    
        int n=nums.size();
	    vector<pair<int, int>> arr;
        
	    for(int i = 0; i < n; i++)
	    {
	        arr.push_back({nums[i], i});
	    }
	    
	    vector<int> count(n, 0);
	    
	    mergeSort(0, n - 1, arr, count);
	    
	    return count;
	}
  
};
247856