Published Dec 18, 2021
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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.
Input: n = 1
Output: "1"
Explanation: This is the base case.
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"
- 1 <= n <= 30
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;
};