二叉树的遍历

复习一下之前的知识 巩固一下自己😀

tree的结构

结构

    public class TreeNode
    {
        public int val;
        public TreeNode left;
        public TreeNode right;
        public TreeNode(int val = 0, TreeNode left = null, TreeNode right = null)
        {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }

前序遍历

• 递归

  public void PreorderTraversal(TreeNode root)
  {
      if(root==null) return;
      Console.WriteLine(root.val);//<--
      PreorderTraversal(root.left);
      PreorderTraversal(root.right);
  }
  

• 栈

  public void InorderTraversal(TreeNode root)
  {
      Stack<TreeNode> stack = new Stack<TreeNode>();
      
      while (root.Any())
      {
          TreeNode node=stack.Peek();
          if (node!=null)//入栈时反着来的
          {
              stack.Pop();
              if(node.right!=null) stack.Push(node.right);
              if(node.left!=null) stack.Push(node.left);
              stack.Push(node);
              stack.push(null);
          }else{
              stack.Pop();
              node=stack.Peek();
              stack.Pop();
              Console.WriteLine(node.val);//<--
          }
      }
  }
  

中序遍历

• 递归

  public void InorderTraversal(TreeNode root)
  {
      if(root==null) return;
      PreorderTraversal(root.left);
      Console.WriteLine(root.val);//<--
      PreorderTraversal(root.right);
  }
  

• 栈

  public void InorderTraversal(TreeNode root)
  {
      Stack<TreeNode> stack = new Stack<TreeNode>();
      
      while (stack.Any())
      {
          TreeNode node=stack.Peek();
          if (node!=null)
          {
              stack.Pop();
              if(node.right!=null) stack.Push(node.right);
              stack.Push(node);
              stack.push(null);
              if(node.left!=null) stack.Push(node.left);
          }else{
              stack.Pop();
              node=stack.Peek();
              stack.Pop();
              Console.WriteLine(node.val);//<--
          }
      }
  }
  

后序遍历

• 递归

  public void PostorderTraversal(TreeNode root)
  {
      if(root==null) return;
      PreorderTraversal(root.left);
      PreorderTraversal(root.right);
      Console.WriteLine(root.val);//<--
  }
  

• 栈

  public void InorderTraversal(TreeNode root)
  {
      Stack<TreeNode> stack = new Stack<TreeNode>();
      
      while (stack.Any())
      {
          TreeNode node=stack.Peek();
          if (node!=null)//入栈时反着来的
          {
              stack.Pop();
              stack.Push(node);
              stack.push(null);
              
              if(node.right!=null) stack.Push(node.right);
              if(node.left!=null) stack.Push(node.left);
          }else{
              stack.Pop();
              node=stack.Peek();
              stack.Pop();
              Console.WriteLine(node.val);
          }
      }
  }
  

层序遍历

• 利用队列(BFS)

  public void InorderTraversal(TreeNode root)
  {
      Queue<TreeNode> queue = new Queue<TreeNode>();
            queue.Enqueue(tree);
            int depth = 1;
            while (queue.Count>0)
            {
                var len = queue.Count;
                for (int i = 0; i < len; i++)
                {
                    var cur = queue.Dequeue();
                    Console.WriteLine("节点 {0} 在第 {1} 层",cur.val, depth);
                    if (cur.left != null)
                        queue.Enqueue(cur.left);
                    if (cur.right != null)
                        queue.Enqueue(cur.right);
                }
                depth++;
            }
  }
  

总结

之前一直用的递归，栈的方法忘了😀，所以栈的方法也是需要总结下的，索性就一起总结一下。

催促下自己去做了这个总结，让自己熟悉一下，不要又忘了，完结 撒花🎉🎉🎉