Is Flip Game II easy or hard?

Flip Game II is generally considered a medium difficulty problem. The recursion logic is straightforward, but recognizing the need for memoization and reasoning about winning versus losing states requires familiarity with backtracking and game theory patterns.

Flip Game II Python/Java solution

In Python or Java, implement a recursive DFS that scans the string for "++" pairs. Replace them with "--", call the function for the opponent’s turn, then restore the string. Use a hash map keyed by the string state to memoize results and avoid recomputation.

How to solve Flip Game II in O(n)?

An exact O(n) general solution is not typically used for this problem because the state space depends on all possible game configurations. The common interview solution uses backtracking with memoization and runs in roughly O(n * 2^n). More advanced theoretical solutions use combinatorial game theory, but they are rarely required in interviews.

What is the best approach for Flip Game II?

Backtracking with memoization is the most effective approach. The algorithm tries every possible "++" flip and recursively checks if the opponent loses from the resulting state. A hash map caches previously evaluated board configurations so each state is computed once. This reduces repeated work across the game tree.

Is Flip Game II asked at Google/Amazon/Meta?

Flip Game II represents a classic game‑theory recursion problem and has appeared in interview preparation sets associated with companies like Google, Amazon, and Meta. Interviewers use it to evaluate reasoning about winning states, recursion, and memoization.

What data structure is used in Flip Game II?

A hash map (or dictionary) is used for memoization to store previously evaluated board states. Recursion explores possible moves, while the map quickly returns results for repeated configurations. Strings or character arrays represent the board state.

What is the time complexity of Flip Game II?

The typical memoized DFS solution runs in O(n * 2^n) time in the worst case. Each unique board configuration may be explored once, and generating next states requires scanning the string for "++" pairs. Space complexity is O(2^n) for recursion and memo storage.

Description Solution Video 6:07

294. Flip Game II

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You are playing a Flip Game with your friend.

You are given a string currentState that contains only '+' and '-'. You and your friend take turns to flip two consecutive "++" into "--". The game ends when a person can no longer make a move, and therefore the other person will be the winner.

Return true if the starting player can guarantee a win, and false otherwise.

Example 1:

Input: currentState = "++++"
Output: true
Explanation: The starting player can guarantee a win by flipping the middle "++" to become "+--+".

Example 2:

Input: currentState = "+"
Output: false

Constraints:

1 <= currentState.length <= 60
currentState[i] is either '+' or '-'.
There cannot be more than 20 consecutive '+'.

Follow up: Derive your algorithm's runtime complexity.

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294. Flip Game II

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