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花花酱 LeetCode 1758. Minimum Changes To Make Alternating Binary String

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".


  • 1 <= s.length <= 104
  • s[i] is either '0' or '1'.

Solution: Two Counters

The final string is either 010101… or 101010…
We just need two counters to record the number of changes needed to transform the original string to those two final strings.

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


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