880. Decoded String at Index #

Problem #

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".

Note:

2 <= S.length <= 100
S will only contain lowercase letters and digits 2 through 9.
S starts with a letter.
1 <= K <= 10^9
The decoded string is guaranteed to have less than 2^63 letters.

Problem Summary #

Given an encoded string S. To find the decoded string and write it to a tape, read one character at a time from the encoded string and take the following steps:

If the character read is a letter, write that letter onto the tape.
If the character read is a digit (for example d), the entire current tape will be written repeatedly d-1 more times in total.

Now, for the given encoded string S and index K, find and return the K-th letter in the decoded string.

Solution Approach #

According to the problem statement, when scanning the string and encountering a digit, start repeating the string; recursion can be used here. Note that when repeating the string, you can return once you reach the K-th character. Do not wait until all characters are fully expanded, as this will time out. d can be extremely large.

Code #


package leetcode

func isLetter(char byte) bool {
	if char >= 'a' && char <= 'z' {
		return true
	}
	return false
}

func decodeAtIndex(S string, K int) string {
	length := 0
	for i := 0; i < len(S); i++ {
		if isLetter(S[i]) {
			length++
			if length == K {
				return string(S[i])
			}
		} else {
			if length*int(S[i]-'0') >= K {
				if K%length != 0 {
					return decodeAtIndex(S[:i], K%length)
				}
				return decodeAtIndex(S[:i], length)
			}
			length *= int(S[i] - '0')
		}
	}
	return ""
}