feat(tree): add binary tree dfs in golang

codes/go/chapter_tree/binary_tree_dfs.go

@@ -0,0 +1,63 @@
+// File: binary_tree_dfs.go
+// Created Time: 2022-11-26
+// Author: Reanon (793584285@qq.com)
+
+package chapter_tree
+
+import (
+	. "github.com/krahets/hello-algo/pkg"
+)
+
+// preOrder 前序遍历
+func preOrder(root *TreeNode) (nums []int) {
+	var preOrderHelper func(node *TreeNode)
+	// 匿名函数
+	preOrderHelper = func(node *TreeNode) {
+		if node == nil {
+			return
+		}
+		// 访问优先级：根结点 -> 左子树 -> 右子树
+		nums = append(nums, node.Val)
+		preOrderHelper(node.Left)
+		preOrderHelper(node.Right)
+	}
+	// 函数调用
+	preOrderHelper(root)
+	return
+}
+
+// inOrder 中序遍历
+func inOrder(root *TreeNode) (nums []int) {
+	var inOrderHelper func(node *TreeNode)
+	// 匿名函数
+	inOrderHelper = func(node *TreeNode) {
+		if node == nil {
+			return
+		}
+		// 访问优先级：左子树 -> 根结点 -> 右子树
+		inOrderHelper(node.Left)
+		nums = append(nums, node.Val)
+		inOrderHelper(node.Right)
+	}
+	// 函数调用
+	inOrderHelper(root)
+	return
+}
+
+// postOrder 后序遍历
+func postOrder(root *TreeNode) (nums []int) {
+	var postOrderHelper func(node *TreeNode)
+	// 匿名函数
+	postOrderHelper = func(node *TreeNode) {
+		if node == nil {
+			return
+		}
+		// 访问优先级：左子树 -> 右子树 -> 根结点
+		postOrderHelper(node.Left)
+		postOrderHelper(node.Right)
+		nums = append(nums, node.Val)
+	}
+	// 函数调用
+	postOrderHelper(root)
+	return
+}