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[LeetCode 38] Count and Say

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.

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

Thoughts

  • Typical recursion problem
  • List several basic cases
  • Find out recursion formula

Typescript

function countAndSay(n: number): string {
  // base case
  if (n === 1) {
    return '1'
  }

  // generate n from n - 1
  let str = countAndSay(n - 1)
  let res = ''
  let count = 1
  let ch = str[0]

  for (let i = 1; i < str.length; i++) {
    if (str[i] !== ch) {
      res += `${count}${ch}`
      ch = str[i]
      count = 1
    } else {
      count++
    }
  }
  res += `${count}${ch}`

  return res
}

Reference