Problem

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, 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”

題目的意思就是反覆的計算字元中，從左向右，每種字元的數量和值。
1 -> 1個1，即11。 11 -> 2個1，即21。 21 -> 1個2，1個1，即1211。以此類推。

Java 實現

package com.coderli.leetcode.algorithms.easy
 
;

/**
 * The count-and-say sequence is the sequence of integers with the first five terms as following:
 * <p>
 * 1.     1 <br>
 * 2.     11<br>
 * 3.     21<br>
 * 4.     1211<br>
 * 5.     111221<br>
 * 1 is read off as "one 1" or 11.<br>
 * 11 is read off as "two 1s" or 21.<br>
 * 21 is read off as "one 2, then one 1" or 1211.<br>
 * Given an integer n, generate the nth term of the count-and-say sequence.
 * <p>
 * Note: Each term of the sequence of integers will be represented as a string.
 * <p>
 * Example 1:
 * <p>
 * Input: 1
 * Output: "1"
 * <p>
 * Example 2:
 * <p>
 * Input: 4
 * Output: "1211"
 *
 * @author li.hzh 2017-10-30 13:06
 */
 

public class CountAndSay {

    public static void main(String[] args) {
        System.out.println(new CountAndSay().countAndSay(1));
        System.out.println(new CountAndSay().countAndSay(2));
        System.out.println(new CountAndSay().countAndSay(3));
        System.out.println(new CountAndSay().countAndSay(4));
        System.out.println(new CountAndSay().countAndSay(5));

    }

    public String countAndSay(int n) {
        String result = "1";
        for (int i = 1; i < n; i++) {
            StringBuilder builder = new StringBuilder();
            char pre = result.charAt(0);
            int count = 0;
            for (int k = 0; k < result.length(); k++) {
                if (result.charAt(k) == pre) {
                    count++;
                } else {
                    builder.append(count);
                    builder.append(pre);
                    count = 1;
                    pre = result.charAt(k);
                }
            }
            builder.append(count);
            builder.append(pre);
            result = builder.toString();
        }
        return result;
    }
}

分析

似乎沒什麼好分析的，遍歷字串，統計每種字元的數量，然後把數量和值依次拼接成一個新的字串。