二叉树相关

//前序
      1
 2       3
4  5    6  7

依次递归的顺序 1 2 4 4 4 2 5 5 5 2 1 3 6 6 6 3 7 7 7 3 1
打印的时机问题
打印放在第一次出现 先序
打印第二次出现  中序
打印第三次    后续
public static void preOrder(TreeNode root){
if(root==null)
return;
System.out.print(root.val);//先序
preOrder(root.left);
//System.out.print(root.val);//中序
preOrder(root.right);
//System.out.print(root.val);//后续
}

public static void preOrder(TreeNode root){
if(root==null)
reutrn;
Stack<TreeNode> stack=new Stack<>();
if(root!=null){
  stack.push(root);
while(!stack.isEmty()){
  TreeNode ret=stack.pop();
  System.out.println(ret.val+" ");
if(ret.right!=null){
stack.push(ret.right);
}
if(ret.left!=null){
stack.push(ret.left);
}
}

}
Systm.out.println();
}

public static void InOrder(TreeNode root){
if(root!=null){
Stack<TreeNode >stack-new Stack<>();
while(!isEmpty()||root!=null){
  if(head!=null){
   stack.push(head);
   head=head.left;
}else{
head=stack.pop()
System.out.print(head.val);
head=head.val;
}
}
}
Systm.out.println();
}

public static void PostOrder(TreeNode root){
if(root!=null){
Stack<ListNode> stack1=new Stack<>();
Stack<ListNode>stack2=new Stack<>();

stack1.push(root);
while(!statck.isEmpty()){
   root=stack1.pop();
  stack2.push(root);
if(root.left!=null){
  stack1.push(root.left);
}else if(root.right!=null){
  stack1.push(root.right);
}


}
while(!stack2.isEmpty())
{
System.out.print(stack2.pop().val+" ‘);
}

}
System.out.println();

} 

public  static TreeNode nextNode(TreeNode node){
if(node==null)
 return node;
if(node.right!=null){
//右子树上最左的节点
node=node.right;
while(node.left!=null){
node=node.left;
}
return node;
}else{
  ListNode parent=node.parent;
  while(parent!=null&&parent.left!=node){
  node= parent;
 parent=node.parent;
}
 return parent;
}

} 

public static TreeNode(TreeNode node){
if(node==null)
return null;
if(node.left!=null){//左子树的最右节点

node=node.left;
while(node.right!=null){
 node=node.right;
}
return node;

}

}else{//以当前节点为右节点的根节点
ListNode parent =node.parent;
while(parent!=null&&parent.right!=node){
 node=parent;
parent=node.parent;

}
return parent;

}