Leetcode #390: Elimination Game
In this guide, we solve Leetcode #390 Elimination Game 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 have a list arr of all integers in the range [1, n] sorted in a strictly increasing order. Apply the following algorithm on arr: Starting from left to right, remove the first number and every other number afterward until you reach the end of the list.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Recursion, Math
Intuition
There is a mathematical invariant or formula that directly leads to the result.
Using math avoids unnecessary loops and reduces complexity.
Approach
Derive the formula or update rule, then compute the answer directly.
Handle edge cases like overflow or zero carefully.
Steps:
- Identify the math relationship.
- Compute the result with a loop or formula.
- Handle edge cases.
Example
Input: n = 9
Output: 6
Explanation:
arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
arr = [2, 4, 6, 8]
arr = [2, 6]
arr = [6]
Python Solution
class Solution:
def lastRemaining(self, n: int) -> int:
a1, an = 1, n
i, step, cnt = 0, 1, n
while cnt > 1:
if i % 2:
an -= step
if cnt % 2:
a1 += step
else:
a1 += step
if cnt % 2:
an -= step
cnt >>= 1
step <<= 1
i += 1
return a1
Complexity
The time complexity is O(n) or O(1). The space complexity is O(1).
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.