java版二叉树算法实现

import java.util.ArrayList;
class BinaryTree {
  private static class TreeNode {
    int data;
    TreeNode left;
    TreeNode right;
    TreeNode(int data){
      this.data = data;
      left = null;
      right = null;
    }
  }
  
  static int[] arr = {1,2,3,4,5,6,7,8,9};
  static ArrayList<TreeNode> tree = new ArrayList<TreeNode>();
  private static void creatTree() {
    for(int i = 0; i < arr.length; i++) {
      tree.add(new TreeNode(arr[i]));
    } 

    // 建立父亲与孩子关系
    for(int i = 0; i <= arr.length / 2 - 1; i++) {
      tree.get(i).left = tree.get(i * 2 + 1);
      tree.get(i).right = tree.get(i * 2 + 2);
    }
   // int lastParent = arr.length / 2 - 1;
   // tree.get(lastParent).left = tree.get(lastParent * 2 + 1);
   // if(arr.length % 2 == 1) {
   //   tree.get(lastParent).right = tree.get(lastParent * 2 + 2);
   // }
  }
  private static void preOrder(TreeNode node){
    if(node == null) return;
    System.out.print(node.data + " ");
    preOrder(node.left);
    preOrder(node.right);
  }
             
  private static void inOrder(TreeNode node) {
    if(node == null) return;
    inOrder(node.left);
    System.out.print(node.data + " ");
    inOrder(node.right);
  }

  private static void postOrder(TreeNode node) {
    if(node == null) return;
    postOrder(node.left);
    postOrder(node.right);
    System.out.print(node.data + " ");  
  }

  public static void main(String[] args) {
   creatTree();
   TreeNode root = tree.get(0);
   System.out.println("先序：");
   preOrder(root);
   System.out.println();
   System.out.println("中序：");
   inOrder(root);
   System.out.println();
   System.out.println("后序：");
   postOrder(root);
   System.out.println();



  }
}

以arraylist为数组结构实现的一个完全二叉树，第n个结点的左孩子为2n+1,右孩子为2n+2