https://leetcode.com/problems/random-flip-matrix/

random-flip-matrix.java

class Solution {
    Map<Integer, Integer> shuffle;
    int n;
    Random rand;
    int n_rows;
    int n_cols;
    public Solution(int n_rows, int n_cols) {
        this.n = n_rows * n_cols;
        this.shuffle = new HashMap<>();
        this.rand = new Random();
        this.n_rows = n_rows;
        this.n_cols = n_cols;
    }
    
    public int[] flip() {
        int idx = this.rand.nextInt(n--);
        
        int pick = this.shuffle.getOrDefault(idx, idx);
        
        this.shuffle.put(idx, this.shuffle.getOrDefault(n, n));
        
        //this.shuffle[n] = pick; // no need;
        
        return new int[]{pick / n_cols, pick % n_cols}; 
    }
    
    public void reset() {
        this.n = n_rows * n_cols;
        this.shuffle.clear();
    }
}

/**
 * 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();
 */