数据结构之二叉树

本文讲解二叉树的基本操作：

• 查找节点
• 计算树的高度
• 清空树
• 递归遍历：先序遍历、中序遍历、后序遍历
• 按层遍历

来看一下树的结构：

class TreeNode {
    String value;
    TreeNode left;
    TreeNode right;
    public TreeNode() {
        
    }
    public TreeNode(String value) {
        this.value = value;
    }
}

首先，为了方便后面看到效果，先手动初始化一个有4个节点的二叉树：

Tree tree = new Tree();
TreeNode root = new TreeNode("root");
TreeNode node1 = new TreeNode("ndoe1");
TreeNode node2 = new TreeNode("ndoe2");
TreeNode node3 = new TreeNode("ndoe3");
root.left = node1;
root.right = node2;
node1.left = node3;

查找节点

//查找节点
public TreeNode findNode(TreeNode treeNode, String value) {
  if(null == treeNode)
    return null;

  if(treeNode.value.equals(value))
    return treeNode;

  TreeNode leftNode = findNode(treeNode.left, value);//递归左子树
  TreeNode rightNode = findNode(treeNode.right, value);//递归右子树
  if(leftNode.value.equals(value))
    return leftNode;
  if(rightNode.value.equals(value))
    return rightNode;

  return null;
}

计算树的深度

//计算树的深度
//递归方法
public int deepth(TreeNode treeNode) {
  if(treeNode == null)
    return 0;
  int left = deepth(treeNode.left);
  int right = deepth(treeNode.right);
  return left > right? left + 1: right + 1;
}

清空树

//清空二叉树
public void clearTreeNode(TreeNode treeNode) {
  if(null != treeNode) {
    clearTreeNode(treeNode.left);
    clearTreeNode(treeNode.right);
    treeNode = null;
  }
}

递归遍历

//遍历1 先序遍历
public void showDLR(TreeNode treeNode) {
  if(null != treeNode) {
    showData(treeNode);
    showDLR(treeNode.left);
    showDLR(treeNode.right);
  }
}
//遍历2 中序遍历
public void showLDR(TreeNode treeNode) {
  if(null != treeNode) {
    showLDR(treeNode.left);
    showData(treeNode);
    showLDR(treeNode.right);
  }
}
//遍历3 后序遍历
public void showLRD(TreeNode treeNode) {
  if(null != treeNode) {
    showLRD(treeNode.left);
    showLRD(treeNode.right);
    showData(treeNode);
  }
}

按层遍历

//遍历4 按层遍历 借助队列 先进先出
public void showByLevel(TreeNode treeNode) {
  if(null == treeNode)
    return;

  LinkedList<TreeNode> list = new LinkedList<>();
  TreeNode current;
  list.offer(treeNode);//将根节点入队

  while(!list.isEmpty()) {
    current = list.poll();//队首出队
    showData(current);//打印节点
    if(null != current.left) {
      list.offer(current.left);
    }
    if(null != current.right) {
      list.offer(current.right);
    }
  }
}

运行结果：

树的深度是：3
先序遍历：
root-->ndoe1-->ndoe3-->ndoe2-->
中序遍历：
ndoe3-->ndoe1-->root-->ndoe2-->
后序遍历：
ndoe3-->ndoe1-->ndoe2-->root-->
按层遍历
root-->ndoe1-->ndoe2-->ndoe3-->