Leetcode #1888: Minimum Number of Flips to Make the Binary String Alternating
In this guide, we solve Leetcode #1888 Minimum Number of Flips to Make the Binary String Alternating in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
You are given a binary string s. You are allowed to perform two types of operations on the string in any sequence: Type-1: Remove the character at the start of the string s and append it to the end of the string.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: String, Dynamic Programming, Sliding Window
Intuition
We are looking for a contiguous region that satisfies a constraint, which is a classic sliding-window signal.
Expanding and shrinking the window lets us maintain validity without restarting the scan.
Approach
Grow the window with a right pointer, and shrink from the left only when the constraint is violated.
Track the best window as you go to keep the solution linear.
Steps:
- Expand the right end of the window.
- While invalid, move the left end to restore constraints.
- Update the best window found.
Example
Input: s = "111000"
Output: 2
Explanation: Use the first operation two times to make s = "100011".
Then, use the second operation on the third and sixth elements to make s = "101010".
Python Solution
class Solution:
def minFlips(self, s: str) -> int:
n = len(s)
target = "01"
cnt = sum(c != target[i & 1] for i, c in enumerate(s))
ans = min(cnt, n - cnt)
for i in range(n):
cnt -= s[i] != target[i & 1]
cnt += s[i] != target[(i + n) & 1]
ans = min(ans, cnt, n - cnt)
return ans
Complexity
The time complexity is O(n). The space complexity is O(1) to O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.