2022-02-14 (394. String decoding)

class Solution {
    
    public String decodeString(String s) {
    
        StringBuilder res = new StringBuilder();
        int multi = 0;
        Stack<Integer> stack_multi = new Stack<>();
        Stack<String> stack_res = new Stack<>();
        for(Character c : s.toCharArray()) {
    
            if(c == '[') {
    
                stack_multi.push(multi);
                stack_res.push(res.toString());
                multi = 0;
                res = new StringBuilder();
            }
            else if(c == ']') {
    
                StringBuilder tmp = new StringBuilder();
                int cur_multi = stack_multi.pop();
                for(int i = 0; i < cur_multi; i++) tmp.append(res);
                res = new StringBuilder(stack_res.pop() + tmp);
            }
            else if(c >= '0' && c <= '9') multi = multi * 10 + Integer.parseInt(c + "");
            else res.append(c);
        }
        return res.toString();

    }
}