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0443 - String Compression (Medium)

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

Problem Statement​

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.

You must write an algorithm that uses only constant extra space.

Given an integer array nums, return true if there exists a triple of indices (i, j, k) such that i < j < k and nums[i] < nums[j] < nums[k]. If no such indices exists, return false.

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".

Constraints:

  • 1 <= chars.length <= 2000
  • chars[i] is a lowercase English letter, uppercase English letter, digit, or symbol.

Follow up: Could you implement a solution that runs in O(n) time complexity and O(1) space complexity?

Approach 1: Iterative​

As stated in the problem, find the consecutive repeating characters frequency and once a set of group is found then modify the array with the character, number of occurances in the next index and repeat the process till last.

Since it's input is a char array, if number of occurrance for a character is more than 9 times, place the numbers in an invididual position and move forward.

Written by @vigneshshiv
class Solution {
    public int compress(char[] chars) {
        int i = 0, res = 0; 
        while (i < chars.length) {
            int groupLength = 1;
            while (i + groupLength < chars.length && chars[i + groupLength] == chars[i]) {
                groupLength += 1;
            }
            chars[res++] = chars[i];
            if (groupLength > 1) {
                for (char c : Integer.toString(groupLength).toCharArray()) {
                    chars[res++] = c;
                }
            }
            i += groupLength;
        }
        return res;
    }
}