#519

Medium

LeetCode Problem Workspace

Random Flip Matrix

Design an optimized algorithm to randomly flip an index in a matrix, using hash tables and math for efficient random selection.

hash table math reservoir sampling randomized

Problem Statement

You are given a m x n binary grid matrix where all values are initially set to 0. The goal is to design an algorithm that randomly selects an index (i, j) where matrix[i][j] is 0, then flips it to 1. Every time an index is flipped, the remaining unflipped indices should have an equal chance of being chosen in future flips.

Your task is to implement a Solution class that performs the 'flip' operation efficiently and includes a 'reset' method. The solution should minimize the number of calls to the random function and optimize the time and space complexity of the algorithm.

Examples

Example 1

Input: See original problem statement.

Output: See original problem statement.

Input ["Solution", "flip", "flip", "flip", "reset", "flip"] [[3, 1], [], [], [], [], []] Output [null, [1, 0], [2, 0], [0, 0], null, [2, 0]]

Explanation Solution solution = new Solution(3, 1); solution.flip(); // return [1, 0], [0,0], [1,0], and [2,0] should be equally likely to be returned. solution.flip(); // return [2, 0], Since [1,0] was returned, [2,0] and [0,0] solution.flip(); // return [0, 0], Based on the previously returned indices, only [0,0] can be returned. solution.reset(); // All the values are reset to 0 and can be returned. solution.flip(); // return [2, 0], [0,0], [1,0], and [2,0] should be equally likely to be returned.

Constraints

1 <= m, n <= 104
There will be at least one free cell for each call to flip.
At most 1000 calls will be made to flip and reset.

Solution Approach

Hash Table for Efficient Index Tracking

A hash table can be used to track the available indices that are still 0. As indices are flipped to 1, they are removed from this table. By keeping track of the available indices and using the hash table for quick lookups, we can efficiently choose random indices to flip.

Reservoir Sampling for Randomness

Reservoir sampling allows for efficient random selection from a dynamic list. When an index is chosen for flipping, the algorithm should randomly select one index from the hash table with equal probability, ensuring that each unflipped index has an equal chance of being chosen.

Optimized Resetting

When the reset function is called, the algorithm should reset the matrix efficiently by restoring all indices to 0. This can be achieved by clearing the hash table that tracks flipped indices, which ensures a fast and simple reset without needing to iterate over the matrix.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time complexity of each flip operation depends on the efficiency of the hash table lookups and random selection process. The space complexity is mainly determined by the size of the hash table, which tracks flipped indices. With at most 1000 flips, the space and time complexities are manageable, but the reset operation is done in constant time, making it efficient for large inputs.

What Interviewers Usually Probe

Can the candidate explain the optimization trade-off between space and time complexity for this problem?
Does the candidate understand the importance of randomness in this problem and how reservoir sampling contributes to it?
Can the candidate implement an efficient reset method without unnecessary iterations over the entire matrix?

Common Pitfalls or Variants

Common pitfalls

Failing to implement uniform randomness in the flip operation, leading to biased index selection.
Using inefficient methods for resetting the matrix, causing excessive time complexity.
Not properly managing the space complexity, leading to memory inefficiencies with large matrices.

Follow-up variants

What if we allow multiple flips of the same index? How would that affect the implementation?
Can this solution be adapted to handle a dynamic size matrix where m and n are continuously changing?
What if the flip function was to be called in a highly concurrent environment? How would you handle synchronization?

FAQ

What is the core pattern for solving the Random Flip Matrix problem?

The core pattern involves using a hash table to track available indices and applying reservoir sampling for efficient random selection, ensuring all unflipped indices have an equal chance of being chosen.

How do I optimize the time complexity for Random Flip Matrix?

Optimizing time complexity involves minimizing unnecessary random function calls and using efficient data structures like hash tables to quickly track available indices for flipping.

Can I use any random function to solve Random Flip Matrix?

No, you need a random function that ensures uniform randomness, and the solution must minimize calls to the random function for efficiency.

How does reservoir sampling help in this problem?

Reservoir sampling allows you to efficiently select a random index from the list of unflipped indices, ensuring each index has an equal probability of being chosen.

What are the space and time complexities for Random Flip Matrix?

The time complexity depends on the hash table operations and random selection, while the space complexity is determined by the size of the hash table, which can hold up to m x n entries.

terminal

Solution

Solution 1

#### Python3

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class Solution:
    def __init__(self, m: int, n: int):
        self.m = m
        self.n = n
        self.total = m * n
        self.mp = {}

    def flip(self) -> List[int]:
        self.total -= 1
        x = random.randint(0, self.total)
        idx = self.mp.get(x, x)
        self.mp[x] = self.mp.get(self.total, self.total)
        return [idx // self.n, idx % self.n]

    def reset(self) -> None:
        self.total = self.m * self.n
        self.mp.clear()


# Your Solution object will be instantiated and called as such:
# obj = Solution(m, n)
# param_1 = obj.flip()
# obj.reset()

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