<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;
    vector<int> numsToAns;
    vector<int> prevNumsToAns;

    void solve(vector<int>& nums, int l, int r) {
        vector<int> left, right;
        int m, lp = 0, rp = 0, idx = l;

        if (l == r) 
            return;

        m = l + (r - l) / 2;
        solve(nums, l, m);
        solve(nums, m + 1, r);
        left = vector<int>(nums.begin() + l, nums.begin() + m + 1);
        right = vector<int>(nums.begin() + m + 1, nums.begin() + r + 1);
        prevNumsToAns = vector<int>(numsToAns.begin() + l, numsToAns.begin() + r + 1);
        while (lp < left.size() || rp < right.size()) {
            if (lp == left.size()) {
                numsToAns[idx] = prevNumsToAns[m + rp + 1 - l];
                nums[idx++] = right[rp++];
            }
            else if (rp == right.size()) {
                numsToAns[idx] = prevNumsToAns[lp];
                ans[numsToAns[idx]] += rp;
                nums[idx++] = left[lp++];
            } else if (left[lp] <= right[rp]) {
                numsToAns[idx] = prevNumsToAns[lp];
                ans[numsToAns[idx]] += rp;
                nums[idx++] = left[lp++];
            } else if (left[lp] > right[rp]) {
                numsToAns[idx] = prevNumsToAns[m + rp + 1 - l];
                nums[idx++] = right[rp++];
            }
        }
    }

    vector<int> countSmaller(vector<int>& nums) {
        int n = nums.size();
        
        ans = vector<int>(n, 0);
        numsToAns = vector<int>(n, 0);
        for (int i = 0; i < n; i++)
            numsToAns[i] = i;
        solve(nums, 0, nums.size() - 1);   

        return ans; 
    }
};
348318