<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 get(char ch) {
        if (ch == 'Q') return 0;
        if (ch == 'E') return 1;
        if (ch == 'W') return 2;
        return 3;
    }

    bool ok(vector<vector<int>>& pref, int n, int l, int r) {
        for (int i = 0; i < 4; ++i) {
            int t = pref[r][i];
            if (l) t -= pref[l - 1][i];
            int cur = pref[n - 1][i] - t;
            if (cur > n / 4) return false;
        }
        return true;
    }

    int balancedString(string s) {
        int n = s.size();
        if (n % 4 != 0) return false;
        vector<vector<int> > pref(n, vector<int> (4, 0));
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < 4; ++j) {
                if (i) pref[i][j] += pref[i - 1][j];
            }
            int code = get(s[i]);
            ++pref[i][code];
        }
        if (pref[n - 1][0] == pref[n - 1][1] && pref[n - 1][0] == pref[n - 1][2] && pref[n - 1][0] == pref[n - 1][3]) return 0;
        int j = 0;
        int answer = 1e9;
        for (int i = 0; i < n; ++i) {
            if (j < i) j = i;
            while (j < n && !ok(pref, n, i, j)) ++j;
            if (j == n) break;
            answer = min(answer, j - i + 1);
        }
        if (answer == 1e9) answer = 0;
        return answer;
    }
};
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