The Challenge
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 writtend-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 = "ha22", K = 5 Output: "h" Explanation: The decoded string is "hahahaha". The 5th letter is "h".
Example 2:
Input: S = "a2345678999999999999999", K = 1 Output: "a" Explanation: The decoded string is "a" repeated 8301530446056247680 times. The 1st letter is "a".
Note:
2 <= S.length <= 100
S
will only contain lowercase letters and digits2
through9
.S
starts with a letter.1 <= K <= 10^9
- The decoded string is guaranteed to have less than
2^63
letters.
The Code
// Our class class Solution { // The entry method public String decodeAtIndex(String S, int K) { long size = 0; int N = S.length(); // Find size = length of decoded string for (int i = 0; i < N; ++i) { char c = S.charAt(i); if (Character.isDigit(c)) size *= c - '0'; else size++; } for (int i = N-1; i >= 0; --i) { char c = S.charAt(i); K %= size; if (K == 0 && Character.isLetter(c)) return Character.toString(c); if (Character.isDigit(c)) size /= c - '0'; else size--; } throw null; } }