Formatted question description: https://leetcode.ca/all/519.html

519. Random Flip Matrix (Medium)

You are given the number of rows n_rows and number of columns n_cols of a 2D binary matrix where all values are initially 0. Write a function flip which chooses a 0 value uniformly at random, changes it to 1, and then returns the position [row.id, col.id] of that value. Also, write a function reset which sets all values back to 0. Try to minimize the number of calls to system's Math.random() and optimize the time and space complexity.

Note:

1 <= n_rows, n_cols <= 10000
0 <= row.id < n_rows and 0 <= col.id < n_cols
flip will not be called when the matrix has no 0 values left.
the total number of calls to flip and reset will not exceed 1000.

Example 1:

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

Example 2:

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

Explanation of Input Syntax:

The input is two lists: the subroutines called and their arguments. Solution's constructor has two arguments, n_rows and n_cols. flip and reset have no arguments. Arguments are always wrapped with a list, even if there aren't any.

Companies:
Google

Related Topics:
Random

Solution 1.

class Solution {
    int n_rows;
    int n_cols;
    int remaining;
    Map<Integer, Integer> map;
    Random random;

    public Solution(int n_rows, int n_cols) {
        this.n_rows = n_rows;
        this.n_cols = n_cols;
        remaining = n_rows * n_cols;
        map = new HashMap<Integer, Integer>();
        random = new Random();
    }

    public int[] flip() {
        int randNum = random.nextInt(remaining);
        remaining--;
        int index = map.getOrDefault(randNum, randNum);
        int value = map.getOrDefault(remaining, remaining);
        map.put(randNum, value);
        int[] rowColumn = {index / n_cols, index % n_cols};
        return rowColumn;
    }

    public void reset() {
        map.clear();
        remaining = n_rows * n_cols;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(n_rows, n_cols);
 * int[] param_1 = obj.flip();
 * obj.reset();
 */
############

class Solution {
    private int m;
    private int n;
    private int total;
    private Random rand = new Random();
    private Map<Integer, Integer> mp = new HashMap<>();

    public Solution(int m, int n) {
        this.m = m;
        this.n = n;
        this.total = m * n;
    }

    public int[] flip() {
        int x = rand.nextInt(total--);
        int idx = mp.getOrDefault(x, x);
        mp.put(x, mp.getOrDefault(total, total));
        return new int[] {idx / n, idx % n};
    }

    public void reset() {
        total = m * n;
        mp.clear();
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(m, n);
 * int[] param_1 = obj.flip();
 * obj.reset();
 */

// OJ: https://leetcode.com/problems/random-flip-matrix/
// Time:
//   Solution: O(MN)
//   flip: O(1)
//   reset: O(1)
// Space: O(MN)
class Solution {
private:
    vector<int> v;
    int size, M, N;
public:
    Solution(int M, int N): M(M), N(N), size(M * N), v(M * N) {
        for (int i = 0; i < size; ++i) v[i] = i;
        srand(time(NULL));
    }
    
    vector<int> flip() {
        swap(v[rand() % size], v[size - 1]);
        --size;
        return { v[size] / N, v[size] % N };
    }
    
    void reset() {
        size = M * N;
    }
};

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()

############

class Solution(object):

    def __init__(self, n_rows, n_cols):
        """
        :type n_rows: int
        :type n_cols: int
        """
        self.M = n_rows
        self.N = n_cols
        self.total = self.M * self.N
        self.fliped = set()

    def flip(self):
        """
        :rtype: List[int]
        """
        pos = random.randint(0, self.total - 1)
        while pos in self.fliped:
            pos = random.randint(0, self.total - 1)
        self.fliped.add(pos)
        return [pos / self.N, pos % self.N]

    def reset(self):
        """
        :rtype: void
        """
        self.fliped.clear()


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

519 - Random Flip Matrix

519. Random Flip Matrix (Medium)

Solution 1.

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519. Random Flip Matrix (Medium)

Solution 1.

All Problems

All Solutions