394. Decode String

Medium

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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".
class Solution {
public:
    string decodeString(string s){
        int index = 0;
        return decode(s,index);
    }
    string decode(string s,int& index){
        string temp = "";
        int Length = s.length();
        while(index < Length && s[index] != ']'){
            if(s[index] < '0' || s[index] > '9'){//--是字母
                temp += s[index++];
            }
            else{
                int cnt = 0;
                while(s[index] >= '0' && s[index] <= '9'){
                    cnt = cnt * 10 + s[index++] - '0';
                }
                index++;//跳过'['
                string t = decode(s,index);
                index++;//跳过']'
                while(cnt-- > 0){
                    temp = temp + t;
                }
            }
        }
        return temp;
    }

};