Introduction
LeetCode 293: Flip Game is a simple problem where you are tasked with flipping a part of a string, represented by a series of + and - symbols. Specifically, you are given a string that contains only + and - characters, and your goal is to flip two consecutive + characters (represented as ++) to --.
The objective is to find all the possible strings you can obtain by flipping one pair of consecutive ++ into -- in each step.
This problem can be approached using a greedy algorithm to explore all possible flips.
Problem Statement
You are given a string s that consists of only + and - characters.
You need to flip two consecutive + characters (++) to -- in the string. Your task is to return a list of all possible strings that can be obtained by doing exactly one flip.
Example
python
Input:
s = "++++"
Output:
["--++", "+--+", "++--"]
Explanation:
There are three possible flips:
1. Flip the first two `+` to `--`, resulting in `--++`.
2. Flip the second and third `+` to `--`, resulting in `+--+`.
3. Flip the last two `+` to `--`, resulting in `++--`.
python
Input:
s = "+"
Output:
[]
Explanation:
There are no consecutive `++` characters in the string, so no flips are possible.
✅ Step-by-Step Solution
🧠 Intuition
The goal is to find all the possible ways to flip two consecutive + signs (++) to --. A simple and efficient way to achieve this is to:
- Iterate through the string to find all instances of
++.
- For each occurrence, flip it to
-- and store the result.
- Return a list of all possible strings formed after each flip.
✅ Approach
- Iterate Over the String:
- Loop through the string
s and look for pairs of consecutive + characters (++).
- Perform the Flip:
- Once you find a pair of
++, replace it with -- to create a new string.
- Store the Results:
- After each flip, store the resulting string in a list.
- Return the List:
- After processing all possible flips, return the list of results.
✅ Python Code
python
class Solution:
def generatePossibleNextMoves(self, s: str):
result = []
for i in range(len(s) - 1):
# Check if we found consecutive '++'
if s[i:i+2] == "++":
# Flip '++' to '--' and store the result
result.append(s[:i] + "--" + s[i+2:])
return result
🧪 How It Works
- Iterate Over the String:
- The loop runs from
i = 0 to i = len(s) - 2 because we are checking pairs of characters, and the last valid index for a pair is len(s) - 2.
- Find
++ and Flip:
- When the substring
s[i:i+2] is ++, it indicates a pair of consecutive plus signs that can be flipped.
- We then create a new string by concatenating the part before the pair,
--, and the part after the pair. This represents the flipped state of the string.
- Store and Return the Result:
- We append each flipped string to the
result list and return it after processing the entire string.
⏱️ Time and Space Complexity
MetricValueTime ComplexityO(n), where n is the length of the string s. The loop iterates through the string once, and checking the substring takes constant time.Space ComplexityO(n), where n is the length of the string s. The space complexity comes from storing the flipped strings in the result list.
- Time Complexity: The loop runs for each character in the string, making it O(n), where
n is the length of the string s.
- Space Complexity: The space complexity is O(n) since, in the worst case, we might need to store the result strings, which could be of length
n.
🔍 Edge Cases
- No Consecutive
++: If there are no consecutive ++ in the string, the result should be an empty list.
- For example, for the input
"--", the result should be [].
- Single
+: If the string contains only one +, there are no pairs of ++ to flip, so the result should be an empty list.
- For example, for the input
"+", the result should be [].
- All
+ Characters: If the entire string consists of only + characters, we should flip every consecutive ++ pair and return all possible results.
- For example, for the input
"++++", the result should be ["--++", "+--+", "++--"].
- Empty String: If the input string is empty, return an empty list, as there are no
++ pairs to flip.
- For example, for the input
"", the result should be [].
✅ Conclusion
LeetCode 293: Flip Game is a problem focused on flipping pairs of consecutive + signs (++) to --. The solution uses a simple approach where we iterate through the string, find every instance of ++, flip it, and store the result. This problem can be solved efficiently in O(n) time complexity, where n is the length of the string, by processing each pair exactly once.
This problem is a great way to practice string manipulation and pattern matching.