38. Count and Say

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

  • countAndSay(1) = "1"
  • countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string.

To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

For example, the saying and conversion for digit string "3322251":

Given a positive integer n, return the nth term of the count-and-say sequence.

 

Example 1:

Input: n = 1
Output: "1"
Explanation: This is the base case.

Example 2:

Input: n = 4
Output: "1211"
Explanation:
countAndSay(1) = "1"
countAndSay(2) = say "1" = one 1 = "11"
countAndSay(3) = say "11" = two 1's = "21"
countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"

 

Constraints:

  • 1 <= n <= 30

Rust Solution

struct Solution;

struct Pair {
    digit: char,
    count: usize,
}

impl Solution {
    fn next(nums: String) -> String {
        let mut prev: Option<Pair> = None;
        let mut s = String::from("");
        for c in nums.chars() {
            if let Some(prev_pair) = prev {
                if prev_pair.digit == c {
                    prev = Some(Pair {
                        digit: c,
                        count: prev_pair.count + 1,
                    });
                } else {
                    s.push_str(&prev_pair.count.to_string());
                    s.push_str(&prev_pair.digit.to_string());
                    prev = Some(Pair { digit: c, count: 1 });
                }
            } else {
                prev = Some(Pair { digit: c, count: 1 });
            }
        }
        if let Some(prev_pair) = prev {
            s.push_str(&prev_pair.count.to_string());
            s.push_str(&prev_pair.digit.to_string());
        }
        s
    }

    fn count_and_say(n: i32) -> String {
        match n {
            1 => String::from("1"),
            2..=30 => Self::next(Solution::count_and_say(n - 1)),
            _ => String::from(""),
        }
    }
}

#[test]
fn test() {
    assert_eq!(Solution::count_and_say(4), String::from("1211"));
    assert_eq!(Solution::count_and_say(1), String::from("1"));
}

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