<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:
    def countSmaller(self, nums: List[int]) -> List[int]:
        def sort(e_l, s, e, r):
            print(s, e)
            # NOTE: 这里不能是e == s退出，而是e - s <= 1
            if e - s <= 1: return
            m = (s + e) // 2
            sort(e_l, s, m, r)
            sort(e_l, m, e, r)
            merge(e_l, s, m, e, r)

        def merge(e_l, s, m, e, r):
            tmp = []
            i = s
            j = m
            s_c = 0 # smaller counter
            while i < m and j < e:
                # NOTE: 不是<而是<=
                if e_l[i][1] <= e_l[j][1]:
                    tmp.append(e_l[i])
                    r[e_l[i][0]] += s_c
                    i += 1
                else:
                    tmp.append(e_l[j])
                    s_c += 1
                    j += 1
            # NOTE: 不要忘记i和j不断++
            while i < m:
                tmp.append(e_l[i])
                r[e_l[i][0]] += s_c
                i += 1
            while j < e:
                tmp.append(e_l[j])
                j += 1
            e_l[s:e] = tmp
        l = len(nums)
        r = [0] * l
        sort(list(enumerate(nums)), 0, l, r)
        print(nums)
        return r
42843