1309. Decrypt String from Alphabet to Integer Mapping.cpp

Given a string s formed by digits ('0' - '9') and '#' . We want to map s to English lowercase characters as follows:

Characters ('a' to 'i') are represented by ('1' to '9') respectively.
Characters ('j' to 'z') are represented by ('10#' to '26#') respectively.
Return the string formed after mapping.

It's guaranteed that a unique mapping will always exist.



Example 1:

Input: s = "10#11#12"
Output: "jkab"
Explanation: "j" -> "10#" , "k" -> "11#" , "a" -> "1" , "b" -> "2".
Example 2:

Input: s = "1326#"
Output: "acz"
Example 3:

Input: s = "25#"
Output: "y"
Example 4:

Input: s = "12345678910#11#12#13#14#15#16#17#18#19#20#21#22#23#24#25#26#"
Output: "abcdefghijklmnopqrstuvwxyz"


Constraints:

1 <= s.length <= 1000
s[i] only contains digits letters ('0'-'9') and '#' letter.
s will be valid string such that mapping is always possible.









class Solution {
public:
    string freqAlphabets(string s) {
        string res;
        int n=s.length();
        for(int i=n-1;i>=0;i--){
            if(s[i]=='#'){
                int index=(s[i-2]-'0')*10 + s[i-1]-'0';
                res+=char('a'-1+index);
                i-=2; // moving two steps
            } else{
                res+=char('a'-1+s[i]-'0');  // -1 as 'a' starts from 1 and not zero
            }
        }
        reverse(res.begin(), res.end());
        return res;
    }
};