38. Count and Say (Easy)

https://leetcode.com/problems/count-and-say/

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

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:

Input: 1
Output: "1"

Example 2:

Input: 4
Output: "1211"

Solutions

class Solution {
    public String countAndSay(int n) {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "1");
        map.put(2, "11");
        map.put(3, "21");
        map.put(4, "1211");
        map.put(5, "111221");

        if (map.keySet().contains(n)) {
            return map.get(n);
        }

        for (int i = 6; i <= n; i++) {
            String pre = map.get(i - 1);

            map.put(i, resolve(pre));
        }

        return map.get(n);
    }


    private String resolve(String s) {
        String ans = "";

        int count = 1;
        char pre = s.charAt(0);

        // 111,22,1
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) != pre) {
                // ans="", count=3, pre=1
                ans += count + "" + pre;

                // reset count
                count = 0;
            }

            // increase count of same char
            count++;
            pre = s.charAt(i);
        }

        ans += count + "" + pre;

        return ans;
    }
}

Incorrect Solutions

References

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