fix some bug and add cap method to array

sparse_array/array3/array3.go

@@ -8,6 +8,8 @@ type Array3 struct {
 	ysize     int
 	xsize     int
 	data      [][][]interface{}
+
+	cap int
 }
 
 func New() *Array3 {
@@ -26,6 +28,8 @@ func NewWithCap(zsize, ysize, xsize int) *Array3 {
 
 	arr.xyproduct = arr.ysize * arr.xsize
 	arr.data = make([][][]interface{}, arr.zsize, arr.zsize)
+
+	arr.cap = arr.zsize * arr.xyproduct
 	return arr
 }
 
@@ -87,8 +91,12 @@ func (arr *Array3) Values() []interface{} {
 	return result
 }
 
+func (arr *Array3) Cap() int {
+	return arr.cap
+}
+
 func (arr *Array3) Grow(size int) {
-	arr.ysize += size
+	arr.zsize += size
 	temp := make([][][]interface{}, arr.zsize, arr.zsize)
 	copy(temp, arr.data)
 	arr.data = temp
@@ -100,6 +108,8 @@ func (arr *Array3) Grow(size int) {
 	tempxsizes := make([][]int, arr.ysize, arr.ysize)
 	copy(tempxsizes, arr.xsizes)
 	arr.xsizes = tempxsizes
+
+	arr.cap = arr.zsize * arr.xyproduct
 }
 
 func (arr *Array3) Set(idx int, value interface{}) {