<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:
    # Sliding Window Solution
    def balancedString(self, s: str) -> int:
        # Create an integer variable, n, representing the length of s, used for easy reference. Create a left pointer, left, initialized to 0. Create an integer variable, min_length, representing the minimum length of the substring that can be replaced with any other string of the same length to make s balanced, initialized to n. Create a Counter variable, s_dict, that keeps track of the characters in s and each character's frequency.
        n = len(s)
        left = 0
        min_length = n
        s_dict = Counter(s)
        # Iterate over the indices of the elements of s using right. At each iteration, decrement s_dict[s[right]] by 1.
        for right in range(n):
            s_dict[s[right]] -= 1
            # While left is in bounds (less than n) and 4 * max(s_dict.values()) is less than or equal to n (no character exceeds the balanced frequency requirement, so the current window forms a balanced substring), update min_length by setting it equal to the minimum of min_length and right - left + 1. Then, move the left pointer forward to try and find a smaller valid substring by incrementing s_dict[s[left]] by 1 and incrementing left by 1.
            while left < n and 4 * max(s_dict.values()) <= n:
                min_length = min(min_length, right - left + 1)
                s_dict[s[left]] += 1
                left += 1
        # After exiting the for loop, return min_length.
        return min_length
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