Random Flip Matrix

class Solution {
    private int R, C;
    private Random random;
    private HashSet<Integer> used;  
    
    public Solution(int n_rows, int n_cols) {
        this.R = n_rows;
        this.C = n_cols;
        this.random = new Random();
        this.used = new HashSet<Integer>();
    }
    
    private int hashCode(int x, int y){
        // Szudzik's Pairing Function
        if(x >= y)
            return x*x + x + y;
        else
            return x + y*y;
    }
    
    public int[] flip() {
        while(true){
            int x = this.random.nextInt(R);
            int y = this.random.nextInt(C);
            int code = hashCode(x, y);
            if(used.add(code)){
                return new int[] { x, y };
            }
        }        
    }
    
    public void reset() {
        this.used.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();
 */