方法一：分治，遞迴

class Solution {
    int index = 0; // 全域性變數記錄遍歷到的位置
    public String decodeString(String s) {
        int n = s.length();
        StringBuilder sb = new StringBuilder();
        int num = 0;
        for(; index < s.length(); index++) {
            if(Character.isDigit(s.charAt(index))) {
                num 
 
= num * 10 + s.charAt(index) - '0'; // 記錄個數
            } else if (s.charAt(index) == '[') {
                index++;
                String str = decodeString(s); // 遇到左括號，遞迴計算這個括號內的解碼字串
                for(int i = 0; i < num; i++) { // 然後新增
                    sb.append(str);
                }
                num 
 
= 0; // 置0
            } else if (s.charAt(index) == ']') {
                return sb.toString(); // 遇到右括號結束遞迴返回給上一層
            } else {
                sb.append(s.charAt(index));// 是字元就新增
            }
        }
        return sb.toString();
    }
}

方法二：利用輔助棧遇到右括號，依次彈出記錄本次括號內的字串

class Solution {
    public
 
 String decodeString(String s) {
        char[] arr = s.toCharArray();
        Stack<Character> stack = new Stack<>();
        for(char c : arr) {
            if(c != ']') {
                stack.push(c);
            } else {
                StringBuilder sb = new StringBuilder();
                while(!stack.isEmpty() && stack.peek() != '[') {
                    sb.append(stack.pop());
                }      
                stack.pop();
                StringBuilder num = new StringBuilder();
                while(!stack.isEmpty() && Character.isDigit(stack.peek())) {
                    num.append(stack.pop());
                }
                int n = Integer.parseInt(num.reverse().toString());
                for(int j = 0; j < n; j++) {
                    for(int i = sb.length() - 1; i >= 0; i--) {
                        stack.push(sb.charAt(i));
                    }
                }
            }
        }
        StringBuilder sb = new StringBuilder();
        while(!stack.isEmpty()) {
            sb.append(stack.pop());
        }
        return sb.reverse().toString();
    }
}