38. Count and Say

Description

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.

For example, the saying and conversion for digit string "3322251":

Given a positive integer n, return the nth term of the count-and-say sequence.

Constraints

• 1 <= n <= 30

Approach

Links

• Binarysearch

• GeeksforGeeks

• Leetcode

• ProgramCreek

• YouTube

Examples

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"

Example 3

Input: n = 10

Output: "13211311123113112211"

Example 4

Input: n = 20

Output: "11131221131211132221232112111312111213111213211231132132211211131221131211221321123113213221123113112221131112311332211211131221131211132211121312211231131112311211232221121321132132211331121321231231121113112221121321133112132112312321123113112221121113122113121113123112112322111213211322211312113211"

Solutions

Solution 1

/**
 * Time complexity : 
 * Space complexity : 
 */

class Solution {
    public String countAndSay(int n) {
        if(n < 2) {
            return n == 0? "": "1";
        }
        StringBuilder result = new StringBuilder("11");
        while(n-- > 2) {
            StringBuilder curr = new StringBuilder();
            int len = result.length(), count = 1;
            for(int i = 1; i < len; i++) {
                if(result.charAt(i-1) == result.charAt(i)) {
                    count++;
                } else {
                    curr.append(count).append(result.charAt(i-1));
                    count = 1;
                }
            }
            curr.append(count).append(result.charAt(len-1));
            result = curr;
        }
        return result.toString();
    }
}

Follow up