394. Decode String Medium

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原文链接：https://leetcode.com/problems/decode-string/

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本文介绍了一种用于解码特定格式字符串的算法，该算法利用字符串栈和操作数栈处理重复子串，通过遍历输入字符串并根据字符类型执行相应操作，实现对复杂嵌套结构的正确解析。

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”.

题目解析：
容器：字符串栈-stringStack、操作数栈-numStack、保存数字的StringBuffer-curNum和保存字符串的StringBuffer-curString。

class Solution {
    public String decodeString(String s) {
        Stack<String> stringStack = new Stack();
        Stack<Integer> numStack = new Stack();
        
        StringBuffer curNum = new StringBuffer();
        StringBuffer curString = new StringBuffer();
        
        char[] arr = s.toCharArray();
        //1.遍历字符串
        for(char c : arr){
        	//2.如果是数字，直接存入curNum。
           if( c >= '0' && c <= '9'){
               curNum.append(c);
            //3.如果是'['，把curNum存入numStack,把curString存入stringStack。并且把curNum和curString置空。
           } else if( c == '['){
               stringStack.push(curString.toString());
               numStack.push(Integer.parseInt(curNum.toString()));
               
               curNum = new StringBuffer();
               curString = new StringBuffer();
            //4.如果是']'，从numStack中取出顶层元素times，把curString复制times次保存到innerStr里。
           } else if( c == ']'){
               int times = numStack.pop();
               StringBuffer innerStr = new StringBuffer();
               
               for(int i = 0; i < times; i++){
                   innerStr.append(curString);
               }
               
               //取出strStack的栈顶元素跟innerStr拼接
               curString = new StringBuffer(stringStack.pop());
               
               curString.append(innerStr);
           } else{
               curString.append(c);
           }
            
        }
        
        return curString.toString();
    }
}