public class TreeNode {
  int val;
  TreeNode left;
  TreeNode right;
  TreeNode(int x){
    val=x;
  }
  @Override
  public String toString(){
    return "val: "+val;
  }
}

  public void visit(TreeNode node){
    System.out.print(node.val+" ");
  }

  /**
   * 遞歸先序遍歷
   * */
  public void preOrderRecursion(TreeNode node){
    if(node==null) //如果結(jié)點(diǎn)為空則返回
      return;
    visit(node);//訪問(wèn)根節(jié)點(diǎn)
    preOrderRecursion(node.left);//訪問(wèn)左孩子
    preOrderRecursion(node.right);//訪問(wèn)右孩子
  }

  /**
   * 非遞歸先序遍歷二叉樹
   * */
  public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> resultList=new ArrayList<>();
    Stack<TreeNode> treeStack=new Stack<>();
    if(root==null) //如果為空樹則返回
      return resultList;
    treeStack.push(root);
    while(!treeStack.isEmpty()){
      TreeNode tempNode=treeStack.pop(); 
      if(tempNode!=null){
        resultList.add(tempNode.val);//訪問(wèn)根節(jié)點(diǎn)
        treeStack.push(tempNode.right); //入棧右孩子
        treeStack.push(tempNode.left);//入棧左孩子
      }
    }
    return resultList;
  }

  /**
   * 遞歸中序遍歷
   * */
  public void preOrderRecursion(TreeNode node){
    if(node==null) //如果結(jié)點(diǎn)為空則返回
      return;
    preOrderRecursion(node.left);//訪問(wèn)左孩子
    visit(node);//訪問(wèn)根節(jié)點(diǎn)
    preOrderRecursion(node.right);//訪問(wèn)右孩子
  }

  /**
   * 非遞歸中序遍歷
   * */
  public List<Integer> inorderTraversalNonCur(TreeNode root) {
    List<Integer> visitedList=new ArrayList<>();
    Map<TreeNode,Integer> visitedNodeMap=new HashMap<>();//保存已訪問(wèn)的節(jié)點(diǎn)
    Stack<TreeNode> toBeVisitedNodes=new Stack<>();//待訪問(wèn)的節(jié)點(diǎn)
    if(root==null)
      return visitedList;
    toBeVisitedNodes.push(root);
    while(!toBeVisitedNodes.isEmpty()){
      TreeNode tempNode=toBeVisitedNodes.peek(); //注意這里是peek而不是pop
      while(tempNode.left!=null){ //如果該節(jié)點(diǎn)的左節(jié)點(diǎn)還未被訪問(wèn)，則需先訪問(wèn)其左節(jié)點(diǎn)
        if(visitedNodeMap.get(tempNode.left)!=null) //該節(jié)點(diǎn)已經(jīng)被訪問(wèn)（不存在某個(gè)節(jié)點(diǎn)已被訪問(wèn)但其左節(jié)點(diǎn)還未被訪問(wèn)的情況）
          break;
        toBeVisitedNodes.push(tempNode.left);
        tempNode=tempNode.left;
      }
      tempNode=toBeVisitedNodes.pop();//訪問(wèn)節(jié)點(diǎn)
      visitedList.add(tempNode.val);
      visitedNodeMap.put(tempNode, 1);//將節(jié)點(diǎn)加入已訪問(wèn)map
      if(tempNode.right!=null) //將右結(jié)點(diǎn)入棧
        toBeVisitedNodes.push(tempNode.right);
    }
    return visitedList;
  }

public List<Integer> inorderTraversal(TreeNode root) {
  List<Integer> list = new ArrayList<Integer>();

  Stack<TreeNode> stack = new Stack<TreeNode>();
  TreeNode cur = root;

  while(cur!=null || !stack.empty()){
    while(cur!=null){
      stack.add(cur);
      cur = cur.left;
    }
    cur = stack.pop();
    list.add(cur.val);
    cur = cur.right;
  }

  return list;
}

  /**
   * 非遞歸后序遍歷
   * */
  public List<Integer> postOrderNonCur(TreeNode root){
    List<Integer> resultList=new ArrayList<>();
    if(root==null)
      return resultList;
    Map<TreeNode,Integer> visitedMap=new HashMap<>();
    Stack<TreeNode> toBeVisitedStack=new Stack<>();
    toBeVisitedStack.push(root);
    while(!toBeVisitedStack.isEmpty()){
      TreeNode tempNode=toBeVisitedStack.peek(); //注意這里是peek而不是pop
      if(tempNode.left==null && tempNode.right==null){ //如果沒(méi)有左右孩子則訪問(wèn)
        resultList.add(tempNode.val);
        visitedMap.put(tempNode, 1);
        toBeVisitedStack.pop();
        continue;
      }else if(!((tempNode.left!=null&&visitedMap.get(tempNode.left)==null )|| (tempNode.right!=null && visitedMap.get(tempNode.right)==null))){
        //如果節(jié)點(diǎn)的左右孩子均已被訪問(wèn)      
        resultList.add(tempNode.val);
        toBeVisitedStack.pop();
        visitedMap.put(tempNode, 1);
        continue;
      }
      if(tempNode.left!=null){
        while(tempNode.left!=null && visitedMap.get(tempNode.left)==null){//左孩子沒(méi)有被訪問(wèn)
          toBeVisitedStack.push(tempNode.left);
          tempNode=tempNode.left;
        }
      }
      if(tempNode.right!=null){
        if(visitedMap.get(tempNode.right)==null){//右孩子沒(méi)有被訪問(wèn)
          toBeVisitedStack.push(tempNode.right);
        }        
      }
    }
    return resultList;
  }

public List<Integer> postorderTraversal(TreeNode root) {
  Deque<TreeNode> stack = new LinkedList<>();
  stack.push(root);
  List<Integer> ret = new ArrayList<>();
  while (!stack.isEmpty()) {
    TreeNode node = stack.pop();
    if (node != null) {
      ret.add(node.val);
      stack.push(node.left);
      stack.push(node.right);
    }
  }
  Collections.reverse(ret);
  return ret;
} 

  public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> resultList=new ArrayList<>();
    int levelNum=0;//記錄某層具有多少個(gè)節(jié)點(diǎn)
    Queue<TreeNode> treeQueue=new LinkedList<>();
    treeQueue.add(root);
    while(!treeQueue.isEmpty()){
      levelNum=treeQueue.size();
      List<Integer> levelList=new ArrayList<>();
      while(levelNum>0){
        TreeNode tempNode=treeQueue.poll();
        if(tempNode!=null){
          levelList.add(tempNode.val);
          treeQueue.add(tempNode.left); 
          treeQueue.add(tempNode.right);
        }
        levelNum--;
      }
      if(levelList.size()>0) 
        resultList.add(levelList);
    }
    return resultList;    
  }