树的前序遍历

在这种遍历方法中，首先访问根节点，然后访问左子树，最后访问右子树。

我们从A开始，按照前序遍历，首先访问A本身，然后移动到它的左子树B。B也进行前序遍历。这个过程一直持续到所有节点都被访问为止。这棵树的前序遍历输出将是：

A → B → D → E → C → F → G

算法

直到所有节点都被遍历：

Step 1: Visit root node.
Step 2: Recursively traverse left subtree.
Step 3: Recursively traverse right subtree.

示例

class Node{
   int data;
   Node leftNode, rightNode;
   
   Node() {
      leftNode = null;
      rightNode = null;     
      this.data = data;
   }
   Node(int data) {
      leftNode = null;
      rightNode = null;     
      this.data = data;
   }
   int getData() {
      return this.data;	   
   }
   Node getleftNode() {
      return this.leftNode;      
   }
   Node getRightNode() {
      return this.leftNode;      
   }
   void setData(int data) {
      this.data = data; 
   }
   void setleftNode(Node leftNode) {
      this.leftNode = leftNode;      
   }
   void setRightNode(Node rightNode) {
      this.leftNode = rightNode;         
   }
}
public class PreOrderBinaryTree {
   public static void main(String[] args) {
      Node node = new Node(50);
      node.leftNode = new Node(60);
      node.leftNode.leftNode = new Node(45);
      node.leftNode.rightNode = new Node(64);

      node.rightNode = new Node(60);
      node.rightNode.leftNode = new Node(45);
      node.rightNode.rightNode = new Node(64);
      System.out.println("pre-order arrangement of given elements: ");
      preOrder(node);
   }   
   public static void preOrder(Node root) {
      if(root !=null) {
         System.out.println(root.data);
         preOrder(root.leftNode);
         preOrder(root.rightNode);         
      }
   }
}

输出

pre-order arrangement of given elements: 
50
60
45
64
60
45
64