int d = 50;
boolean touched = false;

void setup() {
  size(400,400);
}
void draw() {
  background(255);

  ellipse(width/2, height/2, d, d);
  // increase and decrease ellipse
  if (d < width && touched == false) {
    d++;
  } else if (d > 50 && touched == true) {
    d--;
  }

  // switch between increasing and decreasing
  if (touched == true && d == 50) {
    touched = false;
  }
  if (d == width) {
    touched = true;
  }
}