Decode String

Given an encoded string, return it's 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".

Tips:

建立count和result两个stack，result先push(""),然后判断如下情况：

1. 数字：转化完成后放入count栈代用；
2.  [ ：result入栈”“；
3.  ] ：取出result.pop()的结果，入栈count.pop()次，注意要用StringBuilder, 可变长度；      
    注意最后：result.push(result.pop() + sb.toString());result里之前的内容也要加。
4. 字母：result.push(result.pop() + s.charAt(i));

Code:

public class Solution {
    public String decodeString(String s) {
        if (s == null || s.length() == 0) return s;
        Stack<String> res = new Stack<>();
        Stack<Integer> count = new Stack<>();
        StringBuilder tempCount = new StringBuilder();
        res.push("");
        for (Character c : s.toCharArray()) {
            if (Character.isDigit(c)) {
                tempCount.append(c);
            } else if (c == '[') {
                count.push(Integer.parseInt(tempCount.toString()));
                tempCount = new StringBuilder();
                res.push("");
            } else if (c == ']') {
                String temp = res.pop();
                StringBuilder sb = new StringBuilder();
                int index = count.pop();
                for (int i = 0; i < index; i++) {
                    sb.append(temp);
                }
                res.push(res.pop() + sb.toString());
            } else {
                res.push(res.pop() + c);
            }
        }
        return res.pop();
    }
}

2.

public class Solution {
    public String decodeString(String s) {
        Stack<Integer> count = new Stack<>();
        Stack<String> result = new Stack<>();
        result.push("");
        int i = 0;
        while (i < s.length()) {
           if (s.charAt(i) >= '0' && s.charAt(i) <= '9') {
                int start = i;
                while (s.charAt(i + 1) >= '0' && s.charAt(i + 1) <= '9') {
                    i++;
                }
                count.push(Integer.parseInt(s.substring(start, i + 1)));
            } else if (s.charAt(i) == '[') {
                result.push("");
            } else if (s.charAt(i) == ']') {
                String temp = result.pop();
                StringBuilder sb = new StringBuilder();
                int index = count.pop();
                for (int j = 0; j < index; j++) {
                    sb.append(temp);
                }
                result.push(result.pop() + sb.toString());
            } else {
                result.push(result.pop() + s.charAt(i));
            }
            i++;
        }
        return result.pop();
    }
}