Package g0501_0600.s0519_random_flip_matrix

Class Solution

  • public class Solution
extends Object
    519 - Random Flip Matrix.

    Medium

    There is an m x n binary grid matrix with all the values set 0 initially. Design an algorithm to randomly pick an index (i, j) where matrix[i][j] == 0 and flips it to 1. All the indices (i, j) where matrix[i][j] == 0 should be equally likely to be returned.

    Optimize your algorithm to minimize the number of calls made to the built-in random function of your language and optimize the time and space complexity.

    Implement the Solution class:

    • Solution(int m, int n) Initializes the object with the size of the binary matrix m and n.
    • int[] flip() Returns a random index [i, j] of the matrix where matrix[i][j] == 0 and flips it to 1.
    • void reset() Resets all the values of the matrix to be 0.

    Example 1:

    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.

    • Constructor Detail

      • Solution

        public Solution​(int nRows,
                int nCols)

    • Method Detail

      • flip

        public int[] flip()

      • reset

        public void reset()