README.md

443. String Compression

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:

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:

Input:
["a"]
Output:
Return 1, and the first 1 characters of the input array should be: ["a"]
Explanation:
Nothing is replaced.

Example 3:

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.

Solutions (Python)

1. Two Pointers

class Solution:
    def compress(self, chars: List[str]) -> int:
        r, w_char, w_pos = 0, 0, 0

        for r in range(len(chars)):
            if r == len(chars) - 1 or chars[r] != chars[r + 1]:
                s = chars[w_char] + (str(r - w_char + 1) if r > w_char else "")

                for ch in s:
                    chars[w_pos] = ch
                    w_pos += 1

                w_char = r + 1

        return w_pos