String Compression — LeetCode 443 Python Solution

The problem

Given an array of characters chars, compress it using the following algorithm: Begin with an empty string s. For each group of consecutive repeating characters in chars: If the group's length is 1, append the character to s.

Example

Input

chars = ["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

The groups are "aa", "bb", and "ccc". This compresses to "a2b2c3".

Python solution

class Solution:
    def compress(self, chars: List[str]) -> int:
        i, k, n = 0, 0, len(chars)
        while i < n:
            j = i + 1
            while j < n and chars[j] == chars[i]:
                j += 1
            chars[k] = chars[i]
            k += 1
            if j - i > 1:
                cnt = str(j - i)
                for c in cnt:
                    chars[k] = c
                    k += 1
            i = j
        return k

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 443. String Compression is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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