String Two Pointer Techniques

public boolean isPalindrome(String s) {
    int left = 0, right = s.length() - 1;
    while (left < right) {
        while (left < right && !Character.isLetterOrDigit(s.charAt(left)))  left++;
        while (left < right && !Character.isLetterOrDigit(s.charAt(right))) right--;
        if (Character.toLowerCase(s.charAt(left)) !=
            Character.toLowerCase(s.charAt(right))) return false;
        left++;
        right--;
    }
    return true;
}

public boolean validPalindrome(String s) {
    int left = 0, right = s.length() - 1;
    while (left < right) {
        if (s.charAt(left) != s.charAt(right)) {
            // Try skipping left or right character
            return isPalin(s, left + 1, right) || isPalin(s, left, right - 1);
        }
        left++;
        right--;
    }
    return true;
}

private boolean isPalin(String s, int l, int r) {
    while (l < r) {
        if (s.charAt(l++) != s.charAt(r--)) return false;
    }
    return true;
}

public void reverseString(char[] s) {
    int left = 0, right = s.length - 1;
    while (left < right) {
        char tmp = s[left];
        s[left++] = s[right];
        s[right--] = tmp;
    }
}

public String reverseWords(String s) {
    // Trim, split on whitespace, reverse array, join
    String[] words = s.trim().split("\\s+");
    int left = 0, right = words.length - 1;
    while (left < right) {
        String tmp = words[left];
        words[left++] = words[right];
        words[right--] = tmp;
    }
    return String.join(" ", words);
}

// Merge two sorted strings (like merge sort step)
public String mergeSorted(String a, String b) {
    StringBuilder sb = new StringBuilder();
    int i = 0, j = 0;
    while (i < a.length() && j < b.length()) {
        if (a.charAt(i) <= b.charAt(j)) sb.append(a.charAt(i++));
        else sb.append(b.charAt(j++));
    }
    while (i < a.length()) sb.append(a.charAt(i++));
    while (j < b.length()) sb.append(b.charAt(j++));
    return sb.toString();
}

// Find smallest window in s containing all chars of t
public String minWindow(String s, String t) {
    int[] need = new int[128];
    for (char c : t.toCharArray()) need[c]++;
    int required = t.length();
    int left = 0, min = Integer.MAX_VALUE, start = 0;

    for (int right = 0; right < s.length(); right++) {
        if (need[s.charAt(right)]-- > 0) required--;

        while (required == 0) { // window is valid
            if (right - left + 1 < min) {
                min = right - left + 1;
                start = left;
            }
            if (need[s.charAt(left++)]++ == 0) required++;
        }
    }
    return min == Integer.MAX_VALUE ? "" : s.substring(start, start + min);
}