<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>segtree;
    void update(int node,int start,int end,int pos,int val){
        if(pos>end||pos<start)return;
        if(start==end && start==pos){
            segtree[node]+=val;
            return;
        }
        int mid=(start+end)/2;
        update(2*node,start,mid,pos,val);
        update(2*node+1,mid+1,end,pos,val);
        segtree[node]=segtree[2*node]+segtree[2*node+1];
    }
    int query(int node,int start,int end,int l,int r){
        if(start>r||end<l)return 0;
        if(start==l&&end==r)return segtree[node];
        else if(start>=l&&end<=r)return segtree[node];
        else{
            int mid=(start+end)/2;
            return query(2*node,start,mid,l,r)+query(2*node+1,mid+1,end,l,r);
        }
    }
    vector<int> countSmaller(vector<int>& nums) {
        int n=nums.size();
        segtree.resize(4*n,0);
        vector<pair<int,int>>v(n);
        for(int i=0;i<n;i++){
            v[i]={nums[i],i};
        }
        sort(v.begin(),v.end());
        map<pair<int,int>,int>pos;
        for(int i=0;i<n;i++){
            pos[v[i]]=i;
        }
        vector<int>ans(n);
        for(int i=n-1;i>=0;i--){
            int id=pos[{nums[i],i}];
            int sum = query(1,0,n-1,0,id-1);
            ans[i]=sum;
            update(1,0,n-1,id,1);
        }
        return ans;
    }
};
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