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