题目描述:

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 kis 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 or2[4].

Examples:

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s = "3[a]2[bc]", return "aaabcbc".
s = "3[a2[c]]", return "accaccacc".
s = "2[abc]3[cd]ef", return "abcabccdcdcdef".

对一个字符串进行解码, 该字符串的编码规则是这样的重复次数[重复内容], 由于有可能出现嵌套, 所以我使用递归来处理这个字符串.

对于括号匹配来说, 使用栈来确定与相应左括号匹配的右括号. 要注意可能会出现不重复的串, 这时要直接把它加到返回串的后面而不处理重复次数.

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class Solution {
	string s;
public:
	string decodeString(string str) {
		s = str;
		return decodeStringImpl(0, s.length());
	}

	string decodeStringImpl(int start, int end) {
		string ret;
		int index = start;
		while(index < end && !isDigit(s[index]) && s[index] != ']'){
		    index++;
		}
		ret += s.substr(start, index - start);
		while (index < end) {
		    if(!isDigit(s[index])){
		        int j = index;
		        while(j < end && !isDigit(s[j])) j++;
		        ret += s.substr(index, j - index);
		        index = j;
		    }
		    else{
    			int leftBracket = getInt(index);
    			int repeat = stoi(s.substr(index, leftBracket - index));
    			int rightBracket = findRightBracket(leftBracket);
    			string s = decodeStringImpl(leftBracket + 1, rightBracket);
    			for (int i = 0; i < repeat; i++) {
    				ret += s;
    			}
    			index = rightBracket + 1;
		    }
		}
		return ret;
	}

	int findRightBracket(int index) {
		vector<int> st;
		st.push_back(index);
		int i = index;
		while (!st.empty() && i < s.length() - 1) {
			i++;
			if (s[i] == '[') st.push_back(i);
			else if (s[i] == ']') st.pop_back();
		}
		return i;
	}

	int getInt(int index) {
		while (index < s.length() && isDigit(s[index])) {
			index++;
		}
		return index;
	}
	
	bool isDigit(char ch){
	    return ch <= '9' && ch >= '0';
	}
};