Count and Say

Easy String Facebook

LintCode: https://www.lintcode.com/en/problem/count-and-say/

LeetCode: https://leetcode.com/problems/count-and-say/#/description

Question

The count-and-say sequence is the sequence of integers beginning as follows:

1, 11, 21, 1211, 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 , generate the nth sequence.

Notice
The sequence of integers will be represented as a string.

Example
Given n = 5 , return "111221" .

Thinking

Tested on leetcode, error input (n < 1) always return “1”.

Solution

Java (Passed on leetcode)

public class Solution {
    public String countAndSay(int n) {
        if (n < 1) {
            return "1";
        }
        String say = "1";
        int k = 2;
        while (k <= n) {
            String sayExtra = say + "$"; // add an extra char to help handle last char.
            char pre = say.charAt(0);
            int count = 1;
            String tmpStr = "";
            for (int i = 1; i < sayExtra.length(); i++) {
                if (pre == sayExtra.charAt(i)) {
                    ++count;
                } else {
                    tmpStr = tmpStr + count + pre;
                    count = 1;
                    pre = sayExtra.charAt(i);
                }
            }
            say = tmpStr;
            ++k;
        }
        return say;
    }
}

Java

public class Solution {
    /**
     * @param n the nth
     * @return the nth sequence
     */
    public String countAndSay(int n) {
        // Write your code here
        String cas = "1";
        for (int i = 2; i <= n; i++) {
            String cas2 = "";
            int count = 1;
            char pre = cas.charAt(0);
            for (int j = 1; j < cas.length(); j++) {
                if (pre == cas.charAt(j)) {
                    ++count;
                } else {
                    cas2 = cas2 + count + pre;
                    pre = cas.charAt(j);
                    count = 1;
                }
            }
            cas = cas2 + count + pre;
        }
        return cas;
    }
}