外观数列

[PisecesPeng]

外观数列

"外观数列"是一个整数序列, 从数字1开始, 序列中的每一项都是对前一项的描述.
前五项如下:

1. 1
2. 11
3. 21
4. 1211
5. 111221

1 被读做"one 1" ("一个一"), 即11.
11 被读做"two 1" ("两个一"), 即21.
21 被读做"one 2","one 1" ("一个二", "一个一"), 即1211.

给定正整数n(1 <= b <= 30), 输出外观数列的第n项.
注意: 整数序列中的每一项将表示为一个字符串.

示例1:
输入: 1
输出: "1"
解释: 这是一个基本样例

示例2:
输入: 4
输出: "1211"
解释: 
当n=3时, 序列是"21", 其中我们有"2"和"1"两组,
"2"可以读作"12", 也就是出现 频次 = 1而 值 = 2;
类似"1"可以读作"11".
所以答案是"12"和"11"组合在一起, 也就是"1211".

题目地址: https://leetcode-cn.com/problems/count-and-say/

[PisecesPeng]

解题思路

• 遍历构建描述

代码

public static String func(int n) {
    String s = "1";
    for (int j = 1; j < n; j++) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            // 若i为最后一个下标, 则直接返回"1"+"下标值"
            if ((i + 1) >= s.length()) sb.append("1").append(s.charAt(i));
            else {
                int combo = 1;  // 连续出现的字符数
                // 在i不超过最大长度前提下, 一直向后查找相同的字符
                while ((i + 1) < s.length() && Objects.equals(s.charAt(i), s.charAt(i + 1))) {
                    i++;
                    combo++;
                }
                sb.append(combo).append(s.charAt(i));
            }
        }
        s = sb.toString();
    }
    return s;
}

[PisecesPeng]

LeetCode题解

解题思路

• 遍历生成描述

代码

public static String func(int n) {
    String str = "1";
    for (int i = 2; i <= n; ++i) {
        StringBuilder sb = new StringBuilder();
        int start = 0;
        int pos = 0;

        while (pos < str.length()) {
            while (pos < str.length() && str.charAt(pos) == str.charAt(start)) {
                pos++;
            }
            sb.append(Integer.toString(pos - start)).append(str.charAt(start));
            start = pos;
        }
        str = sb.toString();
    }

    return str;
}

[PisecesPeng]

LeetCode题解

解题思路

• 哈希表映射

代码

public static String func(int n) {
    String[] arr = {  "","1","11","21","1211","111221","312211","13112221","1113213211","31131211131221","13211311123113112211","11131221133112132113212221","3113112221232112111312211312113211","1321132132111213122112311311222113111221131221","11131221131211131231121113112221121321132132211331222113112211","311311222113111231131112132112311321322112111312211312111322212311322113212221","132113213221133112132113311211131221121321131211132221123113112221131112311332111213211322211312113211","11131221131211132221232112111312212321123113112221121113122113111231133221121321132132211331121321231231121113122113322113111221131221","31131122211311123113321112131221123113112211121312211213211321322112311311222113311213212322211211131221131211132221232112111312111213111213211231131122212322211331222113112211","1321132132211331121321231231121113112221121321132122311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213122112311311123112111331121113122112132113213211121332212311322113212221","11131221131211132221232112111312111213111213211231132132211211131221131211221321123113213221123113112221131112311332211211131221131211132211121312211231131112311211232221121321132132211331121321231231121113112221121321133112132112312321123113112221121113122113121113123112112322111213211322211312113211","311311222113111231133211121312211231131112311211133112111312211213211312111322211231131122211311122122111312211213211312111322211213211321322113311213212322211231131122211311123113223112111311222112132113311213211221121332211211131221131211132221232112111312111213111213211231132132211211131221232112111312211213111213122112132113213221123113112221131112311311121321122112132231121113122113322113111221131221","132113213221133112132123123112111311222112132113311213211231232112311311222112111312211311123113322112132113213221133122112231131122211211131221131112311332211211131221131211132221232112111312111213322112132113213221133112132113221321123113213221121113122123211211131221222112112322211231131122211311123113321112131221123113111231121113311211131221121321131211132221123113112211121312211231131122211211133112111311222112111312211312111322211213211321322113311213211331121113122122211211132213211231131122212322211331222113112211","111312211312111322212321121113121112131112132112311321322112111312212321121113122112131112131221121321132132211231131122211331121321232221121113122113121113222123112221221321132132211231131122211331121321232221123113112221131112311332111213122112311311123112112322211211131221131211132221232112111312211322111312211213211312111322211231131122111213122112311311221132211221121332211213211321322113311213212312311211131122211213211331121321123123211231131122211211131221131112311332211213211321223112111311222112132113213221123123211231132132211231131122211311123113322112111312211312111322212321121113122123211231131122113221123113221113122112132113213211121332212311322113212221","3113112221131112311332111213122112311311123112111331121113122112132113121113222112311311221112131221123113112221121113311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213213211221113122113121113222112132113213221232112111312111213322112132113213221133112132123123112111311222112132113311213211221121332211231131122211311123113321112131221123113112221132231131122211211131221131112311332211213211321223112111311222112132113212221132221222112112322211211131221131211132221232112111312111213111213211231132132211211131221232112111312211213111213122112132113213221123113112221133112132123222112111312211312112213211231132132211211131221131211132221121311121312211213211312111322211213211321322113311213212322211231131122211311123113321112131221123113112211121312211213211321222113222112132113223113112221121113122113121113123112112322111213211322211312113211","1321132132211331121321231231121113112221121321133112132112312321123113112221121113122113111231133221121321132122311211131122211213211321322112312321123113213221123113112221131112311332211211131221131211132211121312211231131112311211232221121321132132211331121321231231121113121113122122311311222113111231133221121113122113121113221112131221123113111231121123222112111312211312111322212321121113121112131112132112311321322112111312212321121113122122211211232221121321132132211331121321231231121113112221121321132132211322132113213221123113112221133112132123222112111312211312112213211231132132211211131221131211322113321132211221121332211231131122211311123113321112131221123113111231121113311211131221121321131211132221123113112211121312211231131122211211133112111311222112111312211312111322211213211321322123211211131211121332211231131122211311122122111312211213211312111322211231131122211311123113322112111331121113112221121113122113111231133221121113122113121113222123211211131211121332211213211321322113311213212312311211131122211213211321223112111311222112111312211312113221133221121113122113221321132132211231131122211311123113111213211221121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    return arr[n];
}