<p>You are given a string s of length <code>n</code> containing only four kinds of characters: <code>&#39;Q&#39;</code>, <code>&#39;W&#39;</code>, <code>&#39;E&#39;</code>, and <code>&#39;R&#39;</code>.</p>

<p>A string is said to be <strong>balanced</strong><em> </em>if each of its characters appears <code>n / 4</code> times where <code>n</code> is the length of the string.</p>

<p>Return <em>the minimum length of the substring that can be replaced with <strong>any</strong> other string of the same length to make </em><code>s</code><em> <strong>balanced</strong></em>. If s is already <strong>balanced</strong>, return <code>0</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;QWER&quot;
<strong>Output:</strong> 0
<strong>Explanation:</strong> s is already balanced.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;QQWE&quot;
<strong>Output:</strong> 1
<strong>Explanation:</strong> We need to replace a &#39;Q&#39; to &#39;R&#39;, so that &quot;RQWE&quot; (or &quot;QRWE&quot;) is balanced.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;QQQW&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> We can replace the first &quot;QQ&quot; to &quot;ER&quot;. 
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == s.length</code></li>
	<li><code>4 &lt;= n &lt;= 10<sup>5</sup></code></li>
	<li><code>n</code> is a multiple of <code>4</code>.</li>
	<li><code>s</code> contains only <code>&#39;Q&#39;</code>, <code>&#39;W&#39;</code>, <code>&#39;E&#39;</code>, and <code>&#39;R&#39;</code>.</li>
</ul>

class Solution:
    def balancedString(self, s: str) -> int:
        #  QWER
        # maek it so that the count of each of the characters are eequal 
        # have to replace substrings 
        # can we do a "prefix sum" for counts 
        # rolling count 
        
        # [QQQW]
        # [1 , 2 , 3 , 3]
        # [0 , 0 , 0 , 1]
        # [0 , 0 , 0 , 0]
        # [0 , 0 , 0 , 0]

        # upperbound is n
        # lowerbound is 0 
        # find what needs to be added & removed

        # find a substring with the following properties:
        # has Ctr[CHAR]- n//4 surplus characters for all the surplus characters
        # find the smallest substrign like this
        # idea is that we will rebalance it 

        # will the substring with Ctr[CHAR]- n//4 surplus characters always be the one?
            # give it a go 
            # find the smallest such substring using sliding window (first just bruteforce ig just to proof of concept)
        

        n = len(s)
        ctr = Counter(s)
        if ctr["Q"] == ctr["W"] == ctr["E"] == ctr["W"]:
            return 0 
        targetLength = n // 4
        l = 0       
        localcount = {"Q": 0, "W": 0, "E": 0, "R": 0} 
        res = n

        for r in range(n):
            localcount[s[r]] += 1
            
            while l <= r and all(ctr[ch] - localcount[ch] <= targetLength for ch in "QWER"):
                res = min(res, r - l + 1)
                localcount[s[l]] -= 1
                l += 1

        return res
        
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