🔁 Minimum Changes To Make Alternating Binary String

Difficulty: 🟢 Easy
Topics: String, Greedy

📝 Problem Statement

You are given a binary string s consisting only of characters:

• '0'
• '1'

In one operation, you can change:

• '0' → '1'
• OR '1' → '0'

✅ Definition: Alternating String

A string is called alternating if no two adjacent characters are equal.

✔ Example of alternating string:

"010"
"1010"

❌ Not alternating:

"0100"
"1111"

🎯 Task

Return the minimum number of operations required to make the string alternating.

📌 Examples

🔹 Example 1

Input:  s = "0100"
Output: 1

💡 Explanation:
Change last character to '1' → "0101"
Now the string is alternating.

🔹 Example 2

Input:  s = "10"
Output: 0

💡 Explanation:
Already alternating. No changes needed.

🔹 Example 3

Input:  s = "1111"
Output: 2

💡 Explanation:
You can convert it to:

• "0101"
  or
• "1010"

Minimum 2 operations required.

🔒 Constraints

• 1 <= s.length <= 10^4
• s[i] is either '0' or '1'

🧠 Key Insight

There are only two possible alternating patterns:

1. Starting with '0' → "010101..."
2. Starting with '1' → "101010..."

Count mismatches with both patterns and return the minimum.

✨ Perfect for practicing Greedy + String manipulation!

💻 Java Implementation ( Hardcoded Input)

public class Main {

    public static int minOperations(String s) {
        int startWithZero = 0;  // Pattern: 010101...
        int startWithOne = 0;   // Pattern: 101010...

        for (int i = 0; i < s.length(); i++) {

            // Expected characters
            char expectedZero = (i % 2 == 0) ? '0' : '1';
            char expectedOne  = (i % 2 == 0) ? '1' : '0';

            if (s.charAt(i) != expectedZero) {
                startWithZero++;
            }

            if (s.charAt(i) != expectedOne) {
                startWithOne++;
            }
        }

        return Math.min(startWithZero, startWithOne);
    }

    public static void main(String[] args) {

        // Hardcoded Input
        String s = "1111";

        int result = minOperations(s);

        System.out.println("Input: " + s);
        System.out.println("Minimum Operations Required: " + result);
    }
}

⏱ Time Complexity

O(n)

💾 Space Complexity

O(1)

✨ Clean Greedy approach
✨ Only single pass required
✨ Perfect for String pattern understanding