Thinking process

Several things need to remember

• the number of times to repeat of the following enclosed string
• what is the encoded string, what is its start? And what is its end?
• how to get the previous string and attach the repeated current string to it?

Discuss what things need to do if the current character is

• a number => remember this in a variable k
• a '[' => push k to stack and reset the k, push currStr to stack and reset the currStr
• a ']' => append the current k times to previous string

Mistakes

corner cases

• k = 100, but 100 are three separate digits
  • using k = k * 10 + ch - '0'

https://discuss.leetcode.com/topic/57159/simple-java-solution-using-stack/4

public class Solution {
    public String decodeString(String s) {
        StringBuilder currStr = new StringBuilder();

        Stack<Integer> repTimesStack = new Stack<>();
        Stack<StringBuilder> strsStack = new Stack<>();

        int k = 0;
        for (char ch : s.toCharArray()) {
            if (Character.isDigit(ch)) {
                k = k * 10 + ch - '0';
            } else if (ch == '[') {
                strsStack.push(currStr);
                currStr = new StringBuilder();
                repTimesStack.push(k);
                k = 0;
            } else if (ch == ']') {
                StringBuilder tmp = currStr;
                currStr = strsStack.pop();
                for (k = repTimesStack.pop(); k > 0; --k) {
                    currStr.append(tmp);
                }
            } else {
                currStr.append(ch);
            }
        }

        return currStr.toString();
    }
}