2810. Faulty Keyboard

Stack

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class Solution {
    public String finalString(String s) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == 'i') {
                sb.reverse();
            } else {
                sb.append(c);
            }
        }
        return sb.toString();
    }
}

Deque

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class Solution {
    public String finalString(String s) {
        Deque<Character> deque = new ArrayDeque<>(s.length());
        boolean addLast = true;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == 'i') {
                addLast = !addLast;
            } else {
                if (addLast) {
                    deque.addLast(c);
                } else {
                    deque.addFirst(c);
                }
            }
        }

        StringBuilder sb = new StringBuilder();
        for (char c : deque) {
            sb.append(c);
        }
        return addLast ? sb.toString() : sb.reverse().toString();
    }
}

References

2810. Faulty Keyboard