<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 {
public:
    int balancedString(string s) {
        int n = s.size();
        int req = n / 4;
        
        // Count the frequency of characters in the string.
        unordered_map<char, int> mp;
        for (char c : s) mp[c]++;
        
        // Store the excess characters that need balancing.
        unordered_map<char, int> m;
        for (char c : "QWER") {
            if (mp[c] > req) {
                m[c] = mp[c] - req;  // Store the excess of the character.
            }
        }
        
        // If no excess, the string is already balanced.
        if (m.empty()) return 0;
        
        // Sliding window approach.
        unordered_map<char, int> cur;
        int l = 0, min_len = n;
        
        for (int r = 0; r < n; ++r) {
            // Track the current window's character count.
            cur[s[r]]++;
            
            // Check if the current window satisfies the excess condition.
            bool isValid = true;
            for (auto& it : m) {
                if (cur[it.first] < it.second) {
                    isValid = false;
                    break;
                }
            }
            
            // Shrink the window from the left as long as it's valid.
            while (isValid && l <= r) {
                min_len = min(min_len, r - l + 1);
                cur[s[l]]--;
                l++;
                
                // Recheck the condition after shrinking the window.
                for (auto& it : m) {
                    if (cur[it.first] < it.second) {
                        isValid = false;
                        break;
                    }
                }
            }
        }
        
        return min_len;
    }
};

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