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

decode-string.java

class Solution {
    public String decodeString(String s) {
        if (s == null || s.length() == 0) {
            return "";
        }
        return decode(s.toCharArray(), new int[]{0});
    }
    
    public String decode(char[] s, int[] p) {
        StringBuilder result = new StringBuilder();
        while(p[0] < s.length && s[p[0]] != ']') {
            int count = 0;
            while(p[0] < s.length && Character.isDigit(s[p[0]])) {
                count = count * 10 + (s[p[0]] - '0');
                p[0] += 1;
            }
            
            if (s[p[0]] == '['){
                p[0] += 1; // skip /[
                String inbucket = decode(s, p);
                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < count; i++) {
                    sb.append(inbucket);
                }
                result.append(sb);
            } else {
                while(p[0] < s.length && !Character.isDigit(s[p[0]]) && s[p[0]] != ']') {
                    result.append(s[p[0]++]);
                }
            }
        }
        if (p[0] < s.length && s[p[0]] == ']') {
            p[0]++;
        }
        return result.toString();
    }
}