C++     Medium     Recursion     Stack     String    

Problem Statement:

Given an encoded string, return its 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; there are 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 will not be input like 3a or 2[4].

The test cases are generated so that the length of the output will never exceed 105.

 

Example 1:

Input: s = "3[a]2[bc]"
Output: "aaabcbc"

Example 2:

Input: s = "3[a2[c]]"
Output: "accaccacc"

Example 3:

Input: s = "2[abc]3[cd]ef"
Output: "abcabccdcdcdef"

 

Constraints:

  • 1 <= s.length <= 30
  • s consists of lowercase English letters, digits, and square brackets '[]'.
  • s is guaranteed to be a valid input.
  • All the integers in s are in the range [1, 300].

Solution:

Example: 3[a2[c]] After traversal we will have the stack ('',3),(a,2) and have the string c as the current string. From the stack we pop num and then prevString.

 
curString = prevString + num*curString

C++ code:

 
class Solution {
public:
    string decodeString(string s) 
    {
        string curString="", curNum="";
        stack<string> stk;
        for (char ch: s)
        {
            if (ch=='[')
            {
                stk.push(curString);
                stk.push(curNum);
                curString = "";
                curNum = "";
            }
            else if (ch==']')
            {
                int num = stoi(stk.top());
                stk.pop();
                string prevString = stk.top();
                stk.pop();
                string temp = prevString;
                for (int i=0;i<num;i++) 
                    temp += curString;
                curString = temp;
            }
            else if (isdigit(ch))
                curNum += ch;
            else
                curString += ch;
        }
        return curString;
    }
};