38. Count and Say

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

countAndSay(1) = “1”
countAndSay(n) is the way you would “say” the digit string from countAndSay(n-1), which is then converted into a different digit string.
To determine how you “say” a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

Given a positive integer n, return the nth term of the count-and-say sequence.

Example 1:

Input: n = 1
Output: "1"
Explanation: This is the base case.

Example 2:

Input: n = 4
Output: "1211"
Explanation:
countAndSay(1) = "1"
countAndSay(2) = say "1" = one 1 = "11"
countAndSay(3) = say "11" = two 1's = "21"
countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"

如果是給一個數列，前一個數是下個數的個數，例如”3322251”:
個數[2,3,1,1] 數字[3,2,5,1] 最後組成count-and-say sequence[2,3,3,2,1,5,1,1]

那可以用Python的collection包下Counter的类。

from collections import Counter
a = [1, 2, 3, 1, 1, 2]
result = Counter(a)
print result

>>>{1: 3, 2: 2, 3: 1}

不過題目輸入是個數字，一下子想不到，參考一個很漂亮的解法，下次再研究。

---

如果你覺得這篇文章很棒，請你不吝點讚 (ﾟ∀ﾟ)