Solution

public class Solution {
    public String decodeString(String s) {
        Stack<Integer> intS = new Stack<>();
        Stack<StringBuilder> sbS = new Stack<>();
        StringBuilder sb = new StringBuilder();
        int k = 0;
        for(char c : s.toCharArray()) {
            if(Character.isDigit(c)) k = k * 10 + c - '0'; // "k*10" because the number can be > 9 like 125
            else if(c == '[') {
                intS.push(k);
                k = 0;
                sbS.push(sb);
                sb = new StringBuilder();
            } else if(c == ']') {
                StringBuilder temp = sb;
                sb = sbS.pop();
                k = intS.pop();
                while(k-- > 0) sb.append(temp.toString()); //Every time we check the while statement, it will change k even not true 
                k = 0; //So we must set k = 0 otherwise it is "0" after while statement
            } else sb.append(c);
        }
        return sb.toString();
    }
}

Analysis

This is a typical question using Stack. You should always first consider Stack as long as you need to record some data and use it in FILO order. Overall, the question is not very hard but it does need to calculate the data correctly, especially for the K values in while statement and after that.