<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 {
protected:
    void merge_countSmaller(vector<int>& indices, int first, int last, 
                            vector<int>& results, vector<int>& nums) {
        int count = last - first;
        if (count > 1) {
            int step = count / 2;
            int mid = first + step;
            merge_countSmaller(indices, first, mid, results, nums);
            merge_countSmaller(indices, mid, last, results, nums);
            vector<int> tmp;
            tmp.reserve(count);
            int idx1 = first;
            int idx2 = mid;
            int semicount = 0;
            while ((idx1 < mid) || (idx2 < last)) {
                if ((idx2 == last) || ((idx1 < mid) &&
                       (nums[indices[idx1]] <= nums[indices[idx2]]))) {
					tmp.push_back(indices[idx1]);
                    results[indices[idx1]] += semicount;
                    ++idx1;
                } else {
					tmp.push_back(indices[idx2]);
                    ++semicount;
                    ++idx2;
                }
            }
            move(tmp.begin(), tmp.end(), indices.begin()+first);
        }
    }
public:
    vector<int> countSmaller(vector<int>& nums) {
        int n = nums.size();
        vector<int> results(n, 0);
        vector<int> indices(n, 0);
        iota(indices.begin(), indices.end(), 0);
        merge_countSmaller(indices, 0, n, results, nums);
        return results;
    }
};
194981