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Flip Game

Number: 293

Difficulty: Easy

Paid? Yes

Companies: Google

Problem Description

The problem requires generating all possible states of a string after making one valid move. A move consists of flipping two consecutive '+' characters (i.e., "++") to "--". If there is no such pair in the string, the output should be an empty list.

Key Insights

  • Iterate through the string while identifying two consecutive '+' characters.
  • For every valid occurrence of "++", flip it into "--" to create a new state.
  • Use string slicing or substring operations to construct new states.
  • Each transformation is independent, so the order of the output states does not matter.

Space and Time Complexity

Time Complexity: O(N), where N is the length of the string, since we examine the string using a single pass. Space Complexity: O(N) for storing each new string generated (in the worst case, when many valid moves exist).

Solution

The solution involves iterating through the input string from the beginning to the second-to-last character. For each index, check if the current character and the next character are both '+'. If they are, create a new state by concatenating the portion of the string before the current index, the string "--", and the portion after the current index + 2. Append this new state to a results list. The approach employs simple string operations and a linear scan to solve the problem efficiently.

Code Solutions

# Function to generate all valid moves where two consecutive '+' characters are flipped to '--'
def generate_possible_next_moves(currentState):
    results = []  # List to hold all new states
    for i in range(len(currentState) - 1):  # Iterate until the second last character
        # Check for two consecutive '+' characters
        if currentState[i] == '+' and currentState[i+1] == '+':
            # Create new state by flipping '++' to '--'
            new_state = currentState[:i] + "--" + currentState[i+2:]
            results.append(new_state)
    return results

# Example usage
print(generate_possible_next_moves("++++"))
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