(LeetCode) 38. Count and Say

Count and Say

• Explore : Interview > Top Interveiw Questions > Easy Collection
• 분류 : String
• 난이도 : Medium

Problem

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 n^th 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

Solution

Exapnder

class Solution {
    fun countAndSay(n: Int): String {
        if (n == 1) {
            return "1"
        }
        var say = countAndSay(n - 1)

        var result = ""
        var index = 0
        while (index < say.length) {
            var digit = say[index]
            var count: Int = index + 1
            while (count < say.length && say[count] == digit) {
                count++
            }
            result += (count - index).toString().plus(digit)
            index = count
        }
        return result
    }
}

Point of Thinking

• 재귀로 풀면 된다.
• 주어진 n으로 구할 수 있는 문자열을 say, 반복되는 숫자를 count로 처리해서 계속 결합하면 Accpeted.