<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 charmap(char c) {
        if(c == 'Q') return 0;
        else if(c == 'W') return 1;
        else if(c == 'E') return 2;
        else return 3; // if(c == 'R')
    }
    int balancedString(string s) {
        int n = s.size();
        vector<vector<int>> psum(4, vector<int>(n + 1, 0));
        vector<vector<int>> first(4, vector<int>(n + 1, 2*n));

        vector<int> counts = {0, 0, 0, 0};
        for(int i = 0; i < n; i++) {
            for(int cc = 0; cc < 4; cc++) psum[cc][i + 1] = psum[cc][i];
            int charid = charmap(s[i]);
            psum[charid][i + 1]++;
            first[charid][++counts[charid]] = i;
        }
        int ans = n;
        for(int ed = 0; ed <= n; ed++) {
            int st = ed - 1;
            int valid = true;
            for(int ci = 0; ci < 4 && valid; ci++) {
                int rest = psum[ci][n] - psum[ci][ed];
                int allowed = n/4-rest;
                // cout << ci << ": " << rest << ", " << allowed << ", "<<  endl;
                if(allowed < 0) {
                    valid = false;
                    break;
                } else {
                    st = min(st, first[ci][allowed + 1] - 1);
                }
            }
            // cout << " ====" <<  st<< endl;
            if(valid)
                ans = min(ans, (ed - st - 1));
        }
        return ans;


    }
};
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