Run Length Encoding of a String

Problem Statement

Given a string, encode it by counting consecutive repeating characters and appending the character followed by its count.

Characters appearing again later should be treated as a new group.

Input

aaaabbdddeaaff

Output

a4b2d3e1a2f2

Explanation

This is a run-length encoding problem.

We:

• Traverse the string from left to right
• Maintain a count of consecutive characters
• When the character changes, append the previous character and its count
• Reset count for the new character

This handles repeated characters appearing again later correctly.

package exercise.strings;

public class RunLengthEncoding {

    public static String encode(String input) {
        if (input == null || input.isEmpty()) {
            return input;
        }

        StringBuilder result = new StringBuilder();
        int count = 1;

        for (int i = 1; i <= input.length(); i++) {

            if (i < input.length() && input.charAt(i) == input.charAt(i - 1)) {
                count++;
            } else {
                result.append(input.charAt(i - 1))
                        .append(count);
                count = 1;
            }
        }
        return result.toString();
    }

    public static void main(String[] args) {
        System.out.println(encode("aaaabbdddeaaff")); //a4b2d3e1a2f2
    }
}

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Complexity

• Time: O(n)
• Space: O(n)