1758 - Minimum Changes To Make Alternating Binary String (Easy)

Problem Link

https://leetcode.com/problems/minimum-changes-to-make-alternating-binary-string/

Problem Statement

You are given a string s consisting only of the characters '0' and '1'. In one operation, you can change any '0' to '1' or vice versa.

The string is called alternating if no two adjacent characters are equal. For example, the string "010" is alternating, while the string "0100" is not.

Return the minimum number of operations needed to make s alternating.

Example 1:

Input: s = "0100"
Output: 1
Explanation: If you change the last character to '1', s will be "0101", which is alternating.

Example 2:

Input: s = "10"
Output: 0
Explanation: s is already alternating.

Example 3:

Input: s = "1111"
Output: 2
Explanation: You need two operations to reach "0101" or "1010".

Constraints:

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

Approach 1: Counting

Scala

Written by @jit

object Solution {
  // Count swaps required for an alternating string starting with 0.
  // The alternating string starting with 1 is just the complement.
  def minOperations(s: String): Int = {
    val zSwaps = s.indices.count(i => s(i).asDigit == i % 2)
    zSwaps min (s.length - zSwaps)
  }
}

C++

Written by @wingkwong

class Solution {
public:
    int minOperations(string s) {
        int cnt = 0, n = s.size();
        for (int i = 0; i < n; i++) cnt += s[i] - '0' == i % 2;
        return min(cnt, n - cnt);
    }
};