https://leetcode.com/problems/decode-string/

leetcode.java

class Solution {
    private boolean isDigit(String s) {
        return s.matches("\\d");
    }
    
    private String generate(String unit, int times) {
        StringBuilder sb = new StringBuilder();
        while (times > 0) {
            sb.append(unit);
            times--;
        }
        return sb.toString();
    }
    
    public String decodeString(String s) {
        Deque<String> deque = new ArrayDeque<>();
        int i = 0; // current index
        StringBuilder sb = new StringBuilder();
        while(i < s.length()) {
            char c = s.charAt(i);
            if (c == ']') {
                String unit = "", numString = "";
                while(!deque.peek().equals("[")) {
                    unit = deque.pop() + unit;
                }
                deque.pop();
                while(!deque.isEmpty() && isDigit(deque.peek())) {
                    numString = deque.pop() + numString;
                }
                int times = Integer.parseInt(numString);
                deque.push(generate(unit, times));
            } else {
                deque.push(Character.toString(c));
            }
            i++;
        }
        while(!deque.isEmpty()) sb.insert(0, deque.pop());
        return sb.toString();
    }
}