LeetCode每日一题：count and say

问题描述

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...
1is read off as"one 1"or 11.
11is read off as"two 1s"or 21.
21is read off as"one 2, thenone 1"or 1211.
Given an integer n, generate the n th sequence.
Note: The sequence of integers will be represented as a string.

问题分析

这题的题意比较难理解：

• n=0, “1”
• n=1, 上述只有一个1，故 “11”
• n=2, 上述有两个1，故 “21”
• n=3, 上述有一个2、一个1， 故“12 11”
• n=4, 上述有1个1、一个2、两个1，故“11 12 21”
  理解字符串的规律就和容易做了。

代码实现

public String countAndSay(int n) {
        String result = "1";
        for (int i = 1; i < n; i++) {
            result = countString(result);
        }
        return result;
    }

    private String countString(String result) {
        char c = result.charAt(0);
        int count = 1;
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 1; i < result.length(); i++) {
            if (result.charAt(i) == c) {
                count++;
                continue;
            } else {
                stringBuilder.append(String.valueOf(count) + c);
                c = result.charAt(i);
                count = 1;
            }
        }
        stringBuilder.append(String.valueOf(count) + c);
        return stringBuilder.toString();
    }