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
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"));
}