README.md

880. Decoded String at Index

You are given an encoded string s. To decode the 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 d, the entire current tape is repeatedly written d - 1 more times in total.

Given an integer k, return the kth 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 consists of lowercase English letters and digits 2 through 9.
• s starts with a letter.
• 1 <= k <= 109
• It is guaranteed that k is less than or equal to the length of the decoded string.
• The decoded string is guaranteed to have less than 263 letters.

Solutions (Rust)

1. Solution

impl Solution {
    pub fn decode_at_index(s: String, k: i32) -> String {
        let mut k = k as i64 - 1;
        let mut chars = vec![];
        let mut length = 0;

        for ch in s.bytes() {
            chars.push((ch, length));

            if ch.is_ascii_lowercase() {
                length += 1;
            } else {
                length *= (ch - b'0') as i64;
            }

            if length > k {
                break;
            }
        }

        while let Some((ch, i)) = chars.pop() {
            if ch.is_ascii_lowercase() {
                if i == k {
                    return String::from_utf8(vec![ch]).unwrap();
                }

                length -= 1;
            } else {
                length /= (ch - b'0') as i64;
                k %= length;
            }
        }

        unreachable!()
    }
}