update

hard/587.erect-the-fence.go

@@ -0,0 +1,65 @@
+type Points []Point
+
+func (pts Points) Len() int { return len(pts) }
+func (pts Points) Less(i, j int) bool {
+	if pts[i].X != pts[j].X {
+		return pts[i].X < pts[j].X
+	}
+	return pts[i].Y < pts[j].Y
+}
+func (pts Points) Swap(i, j int) { pts[i], pts[j] = pts[j], pts[i] }
+
+/**
+ * Definition for a point.
+ * type Point struct {
+ *     X int
+ *     Y int
+ * }
+ */
+func outerTrees(points []Point) []Point {
+	k, n := 0, len(points)
+	if n == 1 {
+		return points
+	}
+	sort.Sort(Points(points))
+	pts := make([]Point, n)
+	for i := 0; i < n; i++ {
+		for 1 < k { // a x b = dx1*dy2 - dx2*dy1 <= 0
+			dx1, dx2 := pts[k-1].X-pts[k-2].X, points[i].X-pts[k-1].X
+			dy1, dy2 := pts[k-1].Y-pts[k-2].Y, points[i].Y-pts[k-1].Y
+			if dx1*dy2 <= dx2*dy1 {
+				break
+			}
+			k--
+		}
+		pts[k] = points[i]
+		k++
+	}
+	set := make(map[Point]bool)
+	for i := 0; i < k; i++ {
+		set[pts[i]] = true
+	}
+	k = 0
+	for i := n - 1; 0 <= i; i-- {
+		for 1 < k { // a x b = dx1*dy2 - dx2*dy1 <= 0
+			dx1, dx2 := pts[k-1].X-pts[k-2].X, points[i].X-pts[k-1].X
+			dy1, dy2 := pts[k-1].Y-pts[k-2].Y, points[i].Y-pts[k-1].Y
+			if dx1*dy2 <= dx2*dy1 {
+				break
+			}
+			k--
+		}
+		pts[k] = points[i]
+		k++
+	}
+	for i := 0; i < k; i++ {
+		set[pts[i]] = true
+	}
+	res := make([]Point, len(set))
+	k = 0
+	for p, _ := range set {
+		res[k] = p
+		k++
+	}
+	return res
+}