public boolean flip(int i, int j){
        if(i < 0 || i > this.V-1 || j < 0 || j > this.V-1) 
            throw new IllegalArgumentException("index must be at [o, V-1]");
        
        if(this.matrix[i][j]){
            this.matrix[i][j] = false;
            this.E--;
        }else{
            this.matrix[i][j] = true;
            this.E++;
        }
        return this.matrix[i][j];
    }