38. Count and Say[E]

https://leetcode.com/problems/count-and-say/

Description

The count-and-say sequence is the sequence of integers with the first five terms as following:

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

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n where 1 ≤ n ≤ 30, generate the nth term of the count-and-say sequence. You can do so recursively, in other words from the previous member read off the digits, counting the number of digits in groups of the same digit.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

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

Example 2:

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Input: 4
Output: "1211"
Explanation: For n = 3 the term was "21" in which we have two groups "2" and "1", "2" can be read as "12" which means frequency = 1 and value = 2, the same way "1" is read as "11", so the answer is the concatenation of "12" and "11" which is "1211".

Solution

https://leetcode.com/problems/count-and-say/discuss/16015/Please-change-the-misleading-description

https://leetcode.com/problems/count-and-say/discuss/15999/4-5-lines-Python-solutions

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class Solution:
    def countAndSay(self, n: int) -> str:
        res = '1'
        for _ in range(n - 1):
            res = re.sub(r'(.)\1*', lambda n: str(len(n.group(0))) + n.group(1), res)
        return res

443. String Compression[E]

https://leetcode.com/problems/string-compression/

Description

Given an array of characters, compress it in-place.

The length after compression must always be smaller than or equal to the original array.

Every element of the array should be a character (not int) of length 1.

After you are done modifying the input array in-place, return the new length of the array.

Follow up:
Could you solve it using only O(1) extra space?

Example 1:

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Input:
["a","a","b","b","c","c","c"]

Output:
Return 6, and the first 6 characters of the input array should be: ["a","2","b","2","c","3"]

Explanation:
"aa" is replaced by "a2". "bb" is replaced by "b2". "ccc" is replaced by "c3".

Example 2:

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Input:
["a"]

Output:
Return 1, and the first 1 characters of the input array should be: ["a"]

Explanation:
Nothing is replaced.

Example 3:

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Input:
["a","b","b","b","b","b","b","b","b","b","b","b","b"]

Output:
Return 4, and the first 4 characters of the input array should be: ["a","b","1","2"].

Explanation:
Since the character "a" does not repeat, it is not compressed. "bbbbbbbbbbbb" is replaced by "b12".
Notice each digit has it's own entry in the array.

Note:

  1. All characters have an ASCII value in [35, 126].
  2. 1 <= len(chars) <= 1000.

Solution

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class Solution:
    def compress(self, chars: List[str]) -> int:
        anchor = write = 0
        l = len(chars)
        for read in range(l):
            if read == l - 1 or chars[read] != chars[read + 1]:
                # finish reading aabbbcc 6==7 - 1 or a!=b
                chars[write] = chars[anchor] # a b c
                write += 1
                if read > anchor:
                    # check if it is the only one
                    for digit in str(read - anchor + 1):
                        chars[write] = digit
                        write += 1
                anchor = read + 1
        return write

https://leetcode.com/problems/string-compression/solution/