fix some bug and add cap method to array

sparse_array/arrayn/arrayn.go

@@ -23,6 +23,8 @@ type ArrayN struct {
 
 	dimN int
 	data *Node // []*Node
+
+	cap int
 }
 
 func New() *ArrayN {
@@ -40,6 +42,10 @@ func NewWithCap(dims ...int) *ArrayN {
 		arr.product[i] = pvalue
 	}
 	// arr.data = make([]*Node, arr.dims[0], arr.dims[0])
+	arr.cap = 1
+	for _, d := range arr.dims {
+		arr.cap *= d
+	}
 	return arr
 }
 
@@ -107,6 +113,10 @@ func (arr *ArrayN) set(curDim int, curidx int, pdata **Node, parent *Node) (*Nod
 	return arr.set(curDim-1, nidx, &cur.data.([]*Node)[dimindex], cur)
 }
 
+func (arr *ArrayN) Cap() int {
+	return arr.cap
+}
+
 func (arr *ArrayN) Grow(size int) {
 	arr.dims[0] += size
 
@@ -121,6 +131,11 @@ func (arr *ArrayN) Grow(size int) {
 	newData := make([]*Node, arr.dims[0], arr.dims[0])
 	copy(newData, tempdata)
 	arr.data.data = newData
+
+	arr.cap = 1
+	for _, d := range arr.dims {
+		arr.cap *= d
+	}
 }
 
 func (arr *ArrayN) Set(idx int, value interface{}) {