[leetcode-394-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".

思路：

定义一个函数，参数为i和字符串s， 代表从字符串的下标i开始处理，返回处理后的结果。

 string decodeString(const string& s, int& i)
 {
        string res;        
        while (i < s.length() && s[i] != ']')
　　　　 {
            if (!isdigit(s[i]))
                res += s[i++];
            else
　　　　　　  {
                int n = 0;
                while (i < s.length() && isdigit(s[i]))
                    n = n * 10 + s[i++] - '0';                    
                i++; // '['
                string t = decodeString(s, i);
                i++; // ']'                
                while (n-- > 0)
                    res += t;
            }
        }        
        return res;
    }

    string decodeString(string s)
    {
        int i = 0;
        return decodeString(s, i);
    }

参考：

https://discuss.leetcode.com/topic/57228/0ms-simple-c-solution