The right answer must satisfy two conditions:

- the large rectangle area should be equal to the sum of small rectangles
- count of all the points should be even, and that of all the four corner points should be one

```
public boolean isRectangleCover(int[][] rectangles) {
if (rectangles.length == 0 || rectangles[0].length == 0) return false;
int x1 = Integer.MAX_VALUE;
int x2 = Integer.MIN_VALUE;
int y1 = Integer.MAX_VALUE;
int y2 = Integer.MIN_VALUE;
HashSet<String> set = new HashSet<String>();
int area = 0;
for (int[] rect : rectangles) {
x1 = Math.min(rect[0], x1);
y1 = Math.min(rect[1], y1);
x2 = Math.max(rect[2], x2);
y2 = Math.max(rect[3], y2);
area += (rect[2] - rect[0]) * (rect[3] - rect[1]);
String s1 = rect[0] + " " + rect[1];
String s2 = rect[0] + " " + rect[3];
String s3 = rect[2] + " " + rect[3];
String s4 = rect[2] + " " + rect[1];
if (!set.add(s1)) set.remove(s1);
if (!set.add(s2)) set.remove(s2);
if (!set.add(s3)) set.remove(s3);
if (!set.add(s4)) set.remove(s4);
}
if (!set.contains(x1 + " " + y1) || !set.contains(x1 + " " + y2) || !set.contains(x2 + " " + y1) || !set.contains(x2 + " " + y2) || set.size() != 4) return false;
return area == (x2-x1) * (y2-y1);
}
```