<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 SegmentTree {
    vector<int> tree;

public:
    SegmentTree(int n) { tree.resize(4 * n); }
    void updateTree(int idx, int l, int r, int upIdx) {
        if(upIdx<l || upIdx>r) return;
        if (l == r) {
            tree[idx]++;
            return;
        }
        int mid = l + (r - l) / 2;
        updateTree(2 * idx + 1, l, mid, upIdx);
        updateTree(2 * idx + 2, mid + 1, r, upIdx);
        tree[idx] = tree[2 * idx + 1] + tree[2 * idx + 2];
    }

    int queryTree(int idx, int l, int r, int ql, int qr) {
        if (ql > r || qr < l)
            return 0;
        if (ql <= l && r <= qr)
            return tree[idx];
        int mid = l + (r - l) / 2;
        return queryTree(2 * idx + 1, l, mid, ql, qr) +
               queryTree(2 * idx + 2, mid + 1, r, ql, qr);
    }
};

class Solution {
public:
    vector<int> countSmaller(vector<int>& nums) {
        vector<int> ans;
        SegmentTree s(4 * 20001);
        int n = nums.size();
        for (int i = n - 1; i >= 0; i--) {
            int target = nums[i] + 10000;
            s.updateTree(0, 0, 2e4, target);
            ans.push_back(s.queryTree(0, 0, 2e4, 0, target - 1));
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};
179118