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class Solution {
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  public int balancedString(String s) {
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    int n = s.length(), ans = n, excess = 0;
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    int[] cnt = new int[128];
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    cnt['Q'] = cnt['W'] = cnt['E'] = cnt['R'] = -n / 4;
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    for (char ch : s.toCharArray())
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      if (++cnt[ch] == 1) excess++; // if count reaches 1, it is extra and to be removed.
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    if (excess == 0) return 0;
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    for (int i = 0, j = 0; i < n; i++) { // i = window right end, j = window left end
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      if (--cnt[s.charAt(i)] == 0) excess--; // remove letter at index i
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      while (excess == 0) { // if no more excess, then
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        if (++cnt[s.charAt(j)] == 1) excess++; // we put letter at index j back
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        ans = Math.min(i - j + 1, ans);
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        ; // and update ans accordingly
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        j++;
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      }
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    }
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    return ans;
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  }
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}