<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<int> ans;
    void merge(vector<int> &nums, vector<int> &indexes, int low, int mid, int high){
        int count = 0;
        int i = low, j=mid+1;
        vector<int> mergedArray;
        while(i<=mid && j<=high){
            if(nums[indexes[i]] <= nums[indexes[j]]){
                mergedArray.push_back(indexes[i]);
                ans[indexes[i]]+=count;
                i++;
            } else {
                count++;
                mergedArray.push_back(indexes[j]);
                j++;
            }
        }
        while(i<=mid){
            mergedArray.push_back(indexes[i]);
            ans[indexes[i]] += count;
            i++;
        }
        while(j<=high){
            mergedArray.push_back(indexes[j]);
            //ans[indexes[j]] += count;
            j++;
        }
        i=0;
        for(int k=low; k<=high; k++){
            indexes[k] = mergedArray[i]; 
            i++;
        }
    }
    void mergeSort(vector<int> &nums, vector<int> &indexes, int low, int high){
        if(low >= high)
            return;
        int mid = (low + high) / 2;
        mergeSort(nums, indexes, low, mid);
        mergeSort(nums, indexes, mid+1, high);
        merge(nums, indexes, low, mid, high);
    }
    vector<int> countSmaller(vector<int>& nums) {
        int n = nums.size();
        for(int i=0; i<n; i++){
            ans.push_back(0);
        }
        vector<int> indexes;
        for(int i=0; i<n; i++){
            indexes.push_back(i);
        }
        mergeSort(nums, indexes, 0, n-1);
        return ans;
    }
};
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