Leetcode - Count and Say

My code:

public class Solution {
    public String countAndSay(int n) {
        if (n <= 0)
            return null;
        else if (n == 1)
            return "1";
        int count = 1;
        String result = "1";
        while (count < n) {
            int begin = 0;
            int i = begin + 1;
            String temp = "";
            while (i < result.length()) {
                if (result.charAt(i) == result.charAt(i - 1))
                    i++;
                else {
                    temp += Integer.toString(i - begin) + result.charAt(begin);
                    begin = i;
                    i++;
                }
            }
            temp += Integer.toString(i - begin) + result.charAt(begin);
            result = temp;
            count++;
        }
        return result;
    }
    
    public static void main(String[] args) {
        Solution test = new Solution();
        System.out.println(test.countAndSay(2));
        
    }
}

My test result:

Paste_Image.png

这次作业比较有趣。可以科普下什么叫做 count and say number sequence.

https://en.wikipedia.org/wiki/Look-and-say_sequence

Paste_Image.png

也没有什么技术含量。还是简单的扫描字符串。

**
总结： String, scan
**

Anyway, Good luck, Richardo!

My code:

public class Solution {
    public String countAndSay(int n) {
        if (n <= 0)
            return null;
        StringBuilder s = new StringBuilder(String.valueOf(1));
        for (int i = 1; i < n; i++) {
            s = helper(s);
        }
        return s.toString();
    }
    
    private StringBuilder helper(StringBuilder s) {
        if (s == null || s.length() == 0)
            return null;
        StringBuilder ret = new StringBuilder();
        int counter = 1;
        char pre = s.charAt(0);
        for (int i = 1; i < s.length(); i++) {
            char curr = s.charAt(i);
            if (curr != pre) {
                ret.append(String.valueOf(counter) + pre);
                counter = 1;
                pre = curr;
            }
            else {
                counter++;
            }
        }
        if (counter != 0) {
            ret.append(String.valueOf(counter) + pre);
        }
        return ret;
    }
}

一开始拿String做的，排名在 30%左右。换成，StringBuilder之后，排名变成了70%
题目本身并没有什么难度。
今天把zappos的OA做了。感觉不是很难。但是也是提前看了题目之后才说出这样的话的。现在看到string，palindrome，parentheses。。。第一反应就是，DP, GREEDY
其实完全不用这么紧张的。

palindrome 用DP
valid parentheses可以用dp，也可用stack

Anyway, Good luck, Richardo!

My code:

public class Solution {
    HashMap<Integer, String> map = new HashMap<Integer, String>();
    public String countAndSay(int n) {
        if (n <= 0) {
            return null;
        }
        String s = "1";
        for (int i = 1; i < n; i++) {
            s = transform(s);
        }
        return s;
    }
    
    private String transform(String s) {
        StringBuilder ret = new StringBuilder();
        int begin = 0;
        int end = 1;
        while (end <= s.length()) {
            if (end < s.length() && s.charAt(end) == s.charAt(end - 1)) {
                end++;
            }
            else {
                int diff = end - begin;
                ret.append(diff);
                ret.append(s.charAt(begin));
                begin = end;
                end++;
            }
        }
        return ret.toString();
    }
}

reference:
https://discuss.leetcode.com/topic/14543/straightforward-java-solution/2

这道题目我犯傻比了。
我的想法是：
比如输入 Integer 11
我先写个函数，将Integer -> String
11 -> "Two 1"
再写个函数，将String -> Integer
"Tow 1" -> 21
然后我再拿 21 输入第一个函数，以此类推，拿到第n个数

其实完全不用这么复杂。因为我们最后需要输出的，不是 "Two 1"
而是 "21"

所以，直接把 "11" -> "21"
这样，一下就变简单题了。

Anyway, Good luck, Richardo! -- 09/20/2016