leetcode刷题_外观数列

题目描述

这个是我自己的代码，效率不是很高：

class Solution {
    
    
    public String countAndSay(int n) {
    
    
        if(n == 1)
        {
    
    
            return "1";
        }
        String str = countAndSay(n - 1);
        int i = 0;
        String str1 = "";
        while(i < str.length())
        {
    
    
            int j = i + 1;
            int count = 1;
            while(j < str.length() && str.charAt(i) == str.charAt(j))
            {
    
    
                i++;
                j++;
                count++;
            }
            String str2 = String.valueOf(count) + str.charAt(i);
            str1 = str1 + str2;
            i++;
        }
        return str1;
    }
}

后来发现利用StringBuilder很好：

class Solution {
    
    
    public String countAndSay(int n) {
    
    
        if (n == 1) return "1";
        else {
    
    
            String lastStr = countAndSay(n - 1); // 1 2 1 1
            StringBuilder ans = new StringBuilder();
            int i = 0, j = 1, len = lastStr.length();
            while (j < len) {
    
    
                if (lastStr.charAt(i) != lastStr.charAt(j)) {
    
    
                    ans.append(j - i).append(lastStr.charAt(i));
                    i = j;
                }
                j++;
            }
            ans.append(j - i).append(lastStr.charAt(i));
            return ans.toString();
        }
    }
}

修改后的我的：

class Solution {
    
    
    public String countAndSay(int n) {
    
    
        if(n == 1)
        {
    
    
            return "1";
        }
        String str = countAndSay(n - 1);
        int i = 0;
        StringBuilder str1 = new StringBuilder();
        while(i < str.length())
        {
    
    
            int j = i + 1;
            int count = 1;
            while(j < str.length() && str.charAt(i) == str.charAt(j))
            {
    
    
                i++;
                j++;
                count++;
            }
            str1.append(count).append(str.charAt(i));
            i++;
        }
        return str1.toString();
    }
}

看来大规模数据还是需要利用StringBuilder比较好。