394. Decode String

class Solution {
    public String decodeString(String s) {
        Stack<StringBuilder> cacheStack = new Stack<>();
        cacheStack.push(new StringBuilder());
        Stack<Integer> numStack = new Stack<>();

        int num = 0;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c >= '0' && c <= '9') {
                num = num * 10 + (c - '0');
            } else if (c == '[') {
                numStack.push(num);
                num = 0;
                cacheStack.push(new StringBuilder());
            } else if (c == ']') {
                StringBuilder part = cacheStack.pop();
                int count = numStack.pop();
                for (int j = 0; j < count; j++) {
                    cacheStack.peek().append(part);
                }
            } else {
                // c is alpha
                cacheStack.peek().append(c);
            }
        }

        return cacheStack.pop().toString();
    }
}

Reference

394. Decode String