20. Decoded String at Index.cpp

/*
Decoded String at Index
=======================

An encoded string S is given.  To find and write the decoded string to a tape, the encoded string is read one character at a time and the following steps are taken:

If the character read is a letter, that letter is written onto the tape.
If the character read is a digit (say d), the entire current tape is repeatedly written d-1 more times in total.
Now for some encoded string S, and an index K, find and return the K-th letter (1 indexed) in the decoded string.

Example 1:
Input: S = "leet2code3", K = 10
Output: "o"
Explanation: 
The decoded string is "leetleetcodeleetleetcodeleetleetcode".
The 10th letter in the string is "o".

Example 2:
Input: S = "ha22", K = 5
Output: "h"
Explanation: 
The decoded string is "hahahaha".  The 5th letter is "h".

Example 3:
Input: S = "a2345678999999999999999", K = 1
Output: "a"
Explanation: 
The decoded string is "a" repeated 8301530446056247680 times.  The 1st letter is "a".

Constraints:
2 <= S.length <= 100
S will only contain lowercase letters and digits 2 through 9.
S starts with a letter.
1 <= K <= 10^9
It's guaranteed that K is less than or equal to the length of the decoded string.
The decoded string is guaranteed to have less than 2^63 letters.
*/

class Solution
{
public:
  string decodeAtIndex(string S, int K)
  {
    long size = 0;
    int N = S.size();

    for (int i = 0; i < N; ++i)
    {
      if (isdigit(S[i]))
        size *= S[i] - '0';
      else
        size++;
    }

    for (int i = N - 1; i >= 0; --i)
    {
      K %= size;
      if (K == 0 && isalpha(S[i]))
        return (string) "" + S[i];

      if (isdigit(S[i]))
        size /= S[i] - '0';
      else
        size--;
    }
    return "";
  }
};