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class Solution {
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  public String reverseOnlyLetters(String s) {
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    // converting the string to the charArray...
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    char[] ch = s.toCharArray();
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    int start = 0;
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    int end = s.length() - 1;
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    // Storing all the english alphabets in a hashmap so that the searching becomes easy...
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    HashMap<Character, Integer> hash = new HashMap<>();
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    for (int i = 0; i < 26; i++) {
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      hash.put((char) (97 + i), 1);
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    }
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    for (int i = 0; i < 26; i++) {
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      hash.put((char) (65 + i), 1);
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    }
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    // using two while loops ..since the constraints are too less thats why we can prefer nested
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    // loops approach..
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    while (start < end) {
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      // interating untill start pointer reacher a good character
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      while (start < end && !hash.containsKey(ch[start])) {
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        start++;
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      }
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      // iterating untill the end pointer reaches the good character..
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      while (end > start && !hash.containsKey(ch[end])) {
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        end--;
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      }
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      // swapping the array elements..
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      char temp = ch[start];
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      ch[start] = ch[end];
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      ch[end] = temp;
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      start++;
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      end--;
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    }
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    // converting the charArray to the string again..
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    String ans = new String(ch);
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    return ans;
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    // Time Complexity : O(N) (since the loops will run only till the number of charcters in the
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    // string..)
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    // Space Complexity : O(N) since we used hashmap..
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  }
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}