[LC] 394. Decode String

Given an encoded string, return its decoded string.

The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer.

You may assume that the input string is always valid; No extra white spaces, square brackets are well-formed, etc.

Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there won't be input like 3a or 2[4].

Examples:

s = "3[a]2[bc]", return "aaabcbc".
s = "3[a2[c]]", return "accaccacc".
s = "2[abc]3[cd]ef", return "abcabccdcdcdef".

Solution 1: Use 2 stacks

class Solution:
    def decodeString(self, s: str) -> str:
        char_stack = []
        num_stack = []
        res, num, index = '', 0, 0
        while index < len(s):
            char = s[index]
            if char.isdigit():
                num = 0
                while index < len(s) and s[index].isdigit():
                    num = 10 * num + int(s[index])
                    index += 1
                num_stack.append(num)
            elif char == '[':
                char_stack.append(res)
                res = ''
                index += 1
            elif char == ']':
                cur_char_lst = list(char_stack.pop())      
                times = num_stack.pop()
                for _ in range(times):
                    cur_char_lst.append(res)
                res = ''.join(cur_char_lst)
                index += 1
            else:
                res += char
                index += 1
        return res

solution 2: Use 1 stack

class Solution:
    def decodeString(self, s: str) -> str:
        stack = []
        cur_string, cur_num = '', 0
        for c in s:
            if c.isdigit():
                cur_num = 10 * cur_num + int(c)
            elif c == '[':
                stack.append(cur_string)
                stack.append(cur_num)
                cur_string = ''
                cur_num = 0
            elif c == ']':
                prev_num = stack.pop()
                prev_string = stack.pop()
                cur_string = prev_string + prev_num * cur_string
            else:
                cur_string += c
        return cur_string

class Solution {
    public String decodeString(String s) {
        char[] charArr = s.toCharArray();
        LinkedList<Object> stack = new LinkedList<>();
        int num = 0;
        for (char ch: charArr) {
            if (Character.isDigit(ch)) {
                num = 10 * num + (ch - '0');
            } else if (ch == '[') {
                stack.offerFirst(num);
                num = 0;
            } else if (ch == ']') {
                String cur = popBack(stack);
                Integer preNum = (Integer)stack.pop();
                for (int i = 0; i < preNum; i++) {
                    stack.offerFirst(cur);
                }
            } else {
                stack.offerFirst(String.valueOf(ch));
            }
        }
        return popBack(stack);
    }
    
    private String popBack(LinkedList<Object> stack) {
        LinkedList<String> buffer = new LinkedList<>();
        while (!stack.isEmpty() && (stack.peekFirst() instanceof String)) { 
            buffer.offerFirst((String)stack.pollFirst());
        }
        StringBuilder sb = new StringBuilder();
        while (!buffer.isEmpty()) {
            sb.append(buffer.pollFirst());
        }    
        return sb.toString();     
    }
}