[Leetcode] 443. String Compression

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.
• Otherwise, append the character followed by the group's length.

The compressed string s should not be returned separately, but instead be stored in the input character array chars. Note that group lengths that are 10 or longer will be split into multiple characters in chars.

 

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

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

 

Example 1:

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".

Example 2:

Input: chars = ["a"]
Output: Return 1, and the first character of the input array should be: ["a"]
Explanation: The only group is "a", which remains uncompressed since it's a single character.

Example 3:

Input: chars = ["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: The groups are "a" and "bbbbbbbbbbbb". This compresses to "ab12".

Example 4:

Input: chars = ["a","a","a","b","b","a","a"]
Output: Return 6, and the first 6 characters of the input array should be: ["a","3","b","2","a","2"].
Explanation: The groups are "aaa", "bb", and "aa". This compresses to "a3b2a2". Note that each group is independent even if two groups have the same character.

Constraints:

• 1 <= chars.length <= 2000
• chars[i] is a lower-case English letter, upper-case English letter, digit, or symbol.

같은 숫자를 세는 아이디어는 간단했다. consecutive repeating character 를 세는 것이기 때문에 정렬이 된 것으로 간주할 수 있다. 때문에 바로 앞과 뒤 숫자를 비교하는 것만으로 숫자를 세는 것이 가능하다.

 

import java.util.ArrayList;
import java.util.List;

public class StringCompression {

    public int compress(char[] chars) {
        List<Pair> pairList = new ArrayList<>();
        pairList.add(new Pair(chars[0], 1));

        for (int i = 1; i < chars.length; i++) {
            if (chars[i-1] != chars[i]) {
                pairList.add(new Pair(chars[i], 1));
            } else {
                pairList.get(pairList.size()-1).cnt += 1;
            }
        }

        int i = 0;

        for (Pair pair : pairList) {
            chars[i++] = pair.c;
            int cnt = pair.cnt;

            if (cnt > 1)
                for (int j = 0; j < String.valueOf(cnt).length(); j++)
                    chars[i++] = String.valueOf(cnt).charAt(j);
        }

        return i;
    }

    static class Pair {
        char c;
        int cnt;

        public Pair(char c, int cnt) {
            this.c = c;
            this.cnt = cnt;
        }
    }
}