Count and Say

LeetCode 38 | Difficulty: Medium

Medium

Problem Description

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

- `countAndSay(1) = "1"`

- `countAndSay(n)` is the run-length encoding of `countAndSay(n - 1)`.

Run-length encoding (RLE) is a string compression method that works by replacing consecutive identical characters (repeated 2 or more times) with the concatenation of the character and the number marking the count of the characters (length of the run). For example, to compress the string "3322251" we replace "33" with "23", replace "222" with "32", replace "5" with "15" and replace "1" with "11". Thus the compressed string becomes "23321511".

Given a positive integer n, return the n^th element of the count-and-say sequence.

Example 1:

Input: n = 4

Output: "1211"

Explanation:

countAndSay(1) = "1"
countAndSay(2) = RLE of "1" = "11"
countAndSay(3) = RLE of "11" = "21"
countAndSay(4) = RLE of "21" = "1211"

Example 2:

Input: n = 1

Output: "1"

Explanation:

This is the base case.

Constraints:

- `1 <= n <= 30`

Follow up: Could you solve it iteratively?

Topics: String

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Approach

String Processing

Consider character frequency counts, two-pointer approaches, or building strings efficiently. For pattern matching, think about KMP or rolling hash. For palindromes, expand from center or use DP.

When to use

Anagram detection, palindrome checking, string transformation, pattern matching.

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Solutions

Solution 1: C# (Best: 146 ms)

Metric	Value
Runtime	146 ms
Memory	N/A
Date	2017-10-21

Solution

public class Solution {
    public string CountAndSay(int n) {
        if (n == 0) return "";
    string res = "1";
    while (--n!=0)
    {
        StringBuilder cur = new StringBuilder();
        for (int i = 0; i < res.Length; i++)
        {
            int count = 1;
            while ((i + 1 < res.Length) && (res[i] == res[i + 1]))
            {
                count++;
                i++;
            }
            cur.Append(count.ToString()).Append(res[i]);
        }
        res = cur.ToString();
    }
    return res;
    }
}

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Complexity Analysis

Approach	Time	Space
Solution	$O(n)$	$O(1) to O(n)$

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Interview Tips

Key Points

• Discuss the brute force approach first, then optimize. Explain your thought process.
• LeetCode provides 3 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: Create a helper function that maps an integer to pairs of its digits and their frequencies. For example, if you call this function with "223314444411", then it maps it to an array of pairs [[2,2], [3,2], [1,1], [4,5], [1, 2]].

Hint 2: Create another helper function that takes the array of pairs and creates a new integer. For example, if you call this function with [[2,2], [3,2], [1,1], [4,5], [1, 2]], it should create "22"+"23"+"11"+"54"+"21" = "2223115421".

Hint 3: Now, with the two helper functions, you can start with "1" and call the two functions alternatively n-1 times. The answer is the last integer you will obtain.