LeetCode 38. Count and Say

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LeetCode 38. Count and Say

38. Count and Say

Difficulty: Easy

The count-and-say sequence is the sequence of integers with the first five terms as following:

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1\.     1
2\.     11
3\.     21
4\.     1211
5\.     111221

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n where 1 ≤ n ≤ 30, generate the nth term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

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Input: 1
Output: "1"

Example 2:

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Input: 4
Output: "1211"

Solution

Language: Java

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class Solution {
    public String countAndSay(int n) {
        if (n <= 0) {
            return "";
        }
        if (n == 1) {
            return "1";
        }
        String one = "1";
        n--;
        while (n > 0) {
            one = helper(one);
            n--;
        }
        return one;
    }
    
    private String helper(String s) {
        StringBuilder sb = new StringBuilder();
        char cur = s.charAt(0);
        int count = 1;
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) == cur) {
                count++;
            } else {
                sb.append(count);
                sb.append(cur);
                cur = s.charAt(i);
                count = 1;
            }
        }
        sb.append(count);
        sb.append(cur);
        return sb.toString();
    }
}