doc: add demo

README.md

@@ -101,7 +101,6 @@ func main() {
 -   [Validator](#user-content-validator)
 -   [Xerror](#user-content-xerror)
 
-
 <h3 id="algorithm">1. Algorithm package implements some basic algorithm. eg. sort, search. &nbsp; &nbsp; &nbsp; &nbsp;<a href="#index">index</a></h3>
 
 ```go
@@ -587,7 +586,6 @@ import "github.com/duke-git/lancet/v2/datetime"
     [[doc](https://github.com/duke-git/lancet/blob/main/docs/datetime.md#TimestampNano)]
     [[play](https://go.dev/play/p/A9Oq_COrcCF)]
 
-
 <h3 id="datastructure"> 8. Datastructure package constains some common data structure. eg. list, linklist, stack, queue, set, tree, graph. &nbsp; &nbsp; &nbsp; &nbsp;<a href="#index">index</a></h3>
 
 ```go
@@ -871,7 +869,6 @@ import "github.com/duke-git/lancet/v2/maputil"
     [[doc](https://github.com/duke-git/lancet/blob/main/docs/maputil.md#ConcurrentMap_Range)]
     [[play](https://go.dev/play/p/iqcy7P8P0Pr)]
 
-
 <h3 id="mathutil"> 13. Mathutil package implements some functions for math calculation. &nbsp; &nbsp; &nbsp; &nbsp;<a href="#index">index</a></h3>
 
 ```go
@@ -952,6 +949,9 @@ import "github.com/duke-git/lancet/v2/mathutil"
 -   **<big>Sum</big>** : return sum of passed numbers.
     [[doc](https://github.com/duke-git/lancet/blob/main/docs/mathutil.md#Sum)]
     [[play](https://go.dev/play/p/1To2ImAMJA7)]
+-   **<big>Abs</big>** : returns the absolute value of param nubmer.
+    [[doc](https://github.com/duke-git/lancet/blob/main/docs/mathutil.md#Sum)]
+    [[play](https://go.dev/play/p/fsyBh1Os-1d)]
 
 <h3 id="netutil"> 14. Netutil package contains functions to get net information and send http request. &nbsp; &nbsp; &nbsp; &nbsp;<a href="#index">index</a></h3>
 
@@ -1326,7 +1326,6 @@ import "github.com/duke-git/lancet/v2/slice"
     [[doc](https://github.com/duke-git/lancet/blob/main/docs/slice.md#Join)]
     [[play](https://go.dev/play/p/huKzqwNDD7V)]
 
-
 <h3 id="stream"> 19. Stream package implements a sequence of elements supporting sequential and operations. this package is an experiment to explore if stream in go can work as the way java does. its function is very limited. &nbsp; &nbsp; &nbsp; &nbsp;<a href="#index">index</a></h3>
 
 ```go

README_zh-CN.md

@@ -953,6 +953,9 @@ import "github.com/duke-git/lancet/v2/mathutil"
 -   **<big>Sum</big>** : 求传入参数之和。
     [[doc](https://github.com/duke-git/lancet/blob/main/docs/mathutil_zh-CN.md#Sum)]
     [[play](https://go.dev/play/p/1To2ImAMJA7)]
+-   **<big>Abs</big>** : 求绝对值。
+    [[doc](https://github.com/duke-git/lancet/blob/main/docs/mathutil_zh-CN.md#Sum)]
+    [[play](https://go.dev/play/p/fsyBh1Os-1d)]
 
 <h3 id="netutil"> 14. netutil 网络包支持获取 ip 地址，发送 http 请求。&nbsp; &nbsp; &nbsp; &nbsp;<a href="#index">回到目录</a></h3>