LeetCode – Decode String (Java)

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Given an encoded string, return it’s decoded string. For example, given “3[a2[b]]”, return “abbabbabb”.

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

Java Solution

The key to solve this problem is convert the string to a structured data structure and recursively form the return string.

class Solution {
    public String decodeString(String s) {
        Stack<Exp> stack = new Stack<>();
 
        Exp e = new Exp(1);
        stack.push(e);
 
        String num = "";
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c >= '0' && c <= '9') {
                num = num + c;
            } else if (c == '[') {
                if (num.length() == 0)
                    continue;
 
                int value = Integer.parseInt(num);
                num = "";
                Exp exp = new Exp(value);
                stack.push(exp);
            } else if (c == ']') {
                Exp top = stack.pop();
                stack.peek().list.add(top);
            } else {
                stack.peek().list.add(new Exp(c));
            }
        }
 
        Exp root = stack.pop();
 
        return root.getStr();
    }
}
 
class Exp {
    int count;
    List<Exp> list;
    char c;
 
    public Exp(int num) {
        count = num;
        list = new ArrayList<>();
    }
 
    public Exp(char ch) {
        c = ch;
    }
 
    public String getStr() {
        String result = "";
 
        if (list != null) {
            for (int i = 0; i < count; i++) {
                for (Exp e : list) {
                    result += e.getStr();
                }
            }
        } else {
            result += c;
        }
 
        return result;
    }
}

1 thought on “LeetCode – Decode String (Java)”

  1. Solution Using two stacks.

    class Solution {
    public String decodeString(String s) {
    Stack nums=new Stack();
    Stack str =new Stack();
    str.push("");
    int len=s.length();
    int i=0;
    while(i<len){
    String num="";
    while(i<len && '0'<=s.charAt(i) && s.charAt(i)<='9'){
    num=num+s.charAt(i);
    i++;
    }
    if(!num.equals("")){
    nums.push(Integer.valueOf(num));
    }
    else if(s.charAt(i)=='['){
    str.push("[");
    str.push("");
    i++;
    }
    else if(s.charAt(i)==']'){
    String sf="";
    Stack temp =new Stack();
    while( !str.isEmpty()&& !"[".equals(str.peek()) ){
    temp.push(str.pop());
    }
    while( !temp.isEmpty())
    {
    sf+=temp.pop();
    }
    String x= str.pop();
    str.push(getMultipliedString(sf,nums.pop()) );
    i++;
    }
    else{
    if( !str.isEmpty()&& i<len && !"[".equals(str.peek()) ){
    str.push(str.pop()+s.charAt(i));
    }
    i++;
    }

    }//end of main while loop

    String sf="";
    Stack temp =new Stack();
    while( !str.isEmpty()&& !"[".equals(str.peek()) ){
    temp.push(str.pop());
    }
    while( !temp.isEmpty()){
    sf+=temp.pop();
    }
    return sf;
    }
    String getMultipliedString(String x,int n){
    String res="";
    while(n-->0){
    res+=x;
    }
    return res;
    }

    }

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