Minimum Operations to Ensure No Two Adjacent Characters Are Equal

Problem Statement

Given a lowercase character string str of size N, in one operation, any character can be changed into another character.
The goal is to find the minimum number of operations required such that no two adjacent characters are equal.

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Examples

Example 1

Input:

str = "caaab"  

Output:

1  

Explanation:

• Change the second 'a' to any other character (e.g., 'b'), making the string "cabab".
• No adjacent characters are equal.
• Minimum number of operations = 1.

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Example 2

Input:

str = "xxxxxxx"  

Output:

3  

Explanation:

• Replace 'x' at index 1, 3, and 5 with different characters (e.g., 'a', 'b', 'c').
• Resulting string: "xaxbxcx".
• Minimum number of operations = 3.

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Approach

1. Identify contiguous substrings where all characters are the same.
2. For each such substring of length l, at least floor(l / 2) replacements are required.
3. Iterate over the string, grouping consecutive characters together.
4. Compute floor(l / 2) for each group and sum up the values.

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Time Complexity Analysis

• O(N) time complexity as we iterate over the string once.
• O(1) space complexity since we only use a few extra variables.

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Algorithm

1. Initialize operations = 0, which will store the total required operations.
2. Traverse the string to find contiguous segments of the same character.
3. For each segment of length l, compute floor(l / 2) and add it to operations.
4. Return the final operations count.

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