1758. Minimum Changes To Make Alternating Binary String - Explanation

1. Start with Zero and One

class Solution:
    def minOperations(self, s: str) -> int:
        cur = cnt1 = 0
        for c in s:
            if int(c) != cur:
                cnt1 += 1
            cur ^= 1

        cur = 1
        cnt2 = 0
        for c in s:
            if int(c) != cur:
                cnt2 += 1
            cur ^= 1

        return min(cnt1, cnt2)

Time & Space Complexity

Time complexity: $O(n)$
Space complexity: $O(1)$

2. Start with Zero or One

class Solution:
    def minOperations(self, s: str) -> int:
        count = 0

        for i in range(len(s)):
            if i % 2 == 0:
                count += 1 if s[i] == '0' else 0
            else:
                count += 1 if s[i] == '1' else 0

        return min(count, len(s) - count)

Time & Space Complexity

Time complexity: $O(n)$
Space complexity: $O(1)$