序列化和反序列化二叉树

序列化采用前序遍历二叉树，返回一个带有指针信息的字符串。

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/**  * Definition for a binary tree node.  * type TreeNode struct {  *     Val int  *     Left *TreeNode  *     Right *TreeNode  * }  */   type Codec struct {       }   func Constructor() (_ Codec) {     return }   // Serializes a tree to a single string. func (this *Codec) serialize(root *TreeNode) string {     sb := &strings.Builder{}     var dfs func(*TreeNode)     dfs = func(node *TreeNode) {         if node == nil {             sb.WriteString("null,")             return         }         sb.WriteString(strconv.Itoa(node.Val))         sb.WriteByte(',')         dfs(node.Left)         dfs(node.Right)     }     dfs(root)     return sb.String() }   // Deserializes your encoded data to tree. func (this *Codec) deserialize(data string) *TreeNode {        sp:=strings.Split(data,",")     var build func()*TreeNode     build=func()*TreeNode{         if sp[0]=="null"{         sp=sp[1:]         return nil         }         val,_:=strconv.Atoi(sp[0])         sp=sp[1:]         return &TreeNode{val,build(),build()}     }           return build() }     /**  * Your Codec object will be instantiated and called as such:  * ser := Constructor();  * deser := Constructor();  * data := ser.serialize(root);  * ans := deser.deserialize(data);  */

　　力扣第297题。