二叉树遍历

二叉树三种遍历方式
二叉树深度遍历和广度遍历

1,把二叉树的所有节点值遍历并添加到数组

    public void addList(TreeNode node) {
        if (node != null) {
            list.add(node.val);
            if (node.left != null)
                addList(node.left);
            if (node.right != null)
                addList(node.right);
        }
    }

各种遍历

* 先序遍历二叉树（递归）
   * @param node
   */
  public void preOrderTraverse(Node<E> node) {
    System.out.print(node.value + " ");
    if (node.left != null)
      preOrderTraverse(node.left);
    if (node.right != null)
      preOrderTraverse(node.right);
  }
  /**
   * 中序遍历二叉树（递归）
   * @param node
   */
  public void inOrderTraverse(Node<E> node) {
    if (node.left != null)
      inOrderTraverse(node.left);
    System.out.print(node.value + " ");
    if (node.right != null)
      inOrderTraverse(node.right);
  }
  /**
   * 后序遍历二叉树（递归）
   * @param node
   */
  public void postOrderTraverse(Node<E> node) {
    if (node.left != null)
      postOrderTraverse(node.left);
    if (node.right != null)
      postOrderTraverse(node.right);
    System.out.print(node.value + " ");
  }
  /**
   * 先序遍历二叉树（非递归）
   * @param root
   */
  public void preOrderTraverseNoRecursion(Node<E> root) {
    LinkedList<Node<E>> stack = new LinkedList<Node<E>>();
    Node<E> currentNode = null;
    stack.push(root);
    while (!stack.isEmpty()) {
      currentNode = stack.pop();
      System.out.print(currentNode.value + " ");
      if (currentNode.right != null)
        stack.push(currentNode.right);
      if (currentNode.left != null)
        stack.push(currentNode.left);
    }
  }
  /**
   * 中序遍历二叉树（非递归）
   * @param root
   */
  public void inOrderTraverseNoRecursion(Node<E> root) {
    LinkedList<Node<E>> stack = new LinkedList<Node<E>>();
    Node<E> currentNode = root;
    while (currentNode != null || !stack.isEmpty()) {
      // 一直循环到二叉排序树最左端的叶子结点（currentNode是null）
      while (currentNode != null) {
        stack.push(currentNode);
        currentNode = currentNode.left;
      }
      currentNode = stack.pop();
      System.out.print(currentNode.value + " ");
      currentNode = currentNode.right;
    }
  }
  /**
   * 后序遍历二叉树（非递归）
   * @param root
   */
  public void postOrderTraverseNoRecursion(Node<E> root) {
    LinkedList<Node<E>> stack = new LinkedList<Node<E>>();
    Node<E> currentNode = root;
    Node<E> rightNode = null;
    while (currentNode != null || !stack.isEmpty()) {
      // 一直循环到二叉排序树最左端的叶子结点（currentNode是null）
      while (currentNode != null) {
        stack.push(currentNode);
        currentNode = currentNode.left;
      }
      currentNode = stack.pop();
      // 当前结点没有右结点或上一个结点（已经输出的结点）是当前结点的右结点，则输出当前结点
      while (currentNode.right == null || currentNode.right == rightNode) {
        System.out.print(currentNode.value + " ");
        rightNode = currentNode;
        if (stack.isEmpty()) {
          return; //root以输出，则遍历结束
        }
        currentNode = stack.pop();
      }
      stack.push(currentNode); // 还有右结点没有遍历
      currentNode = currentNode.right;
    }
  }
  /**
   * 广度优先遍历二叉树，又称层次遍历二叉树
   * @param node
   */
  public void breadthFirstTraverse(Node<E> root) {
    Queue<Node<E>> queue = new LinkedList<Node<E>>();
    Node<E> currentNode = null;
    queue.offer(root);
    while (!queue.isEmpty()) {
      currentNode = queue.poll();
      System.out.print(currentNode.value + " ");
      if (currentNode.left != null)
        queue.offer(currentNode.left);
      if (currentNode.right != null)
        queue.offer(currentNode.right);
    }
  }

深度遍历计算二叉树的深度

public int maxDepth(Node root) {
        if (root == null) return 0;
        int depth = 1;
        for (Node children : root.children)
            depth = Math.max(depth, maxDepth(children) + 1);
        return depth;
        
    }

非递归实现二叉树三种遍历方式

先序遍历:
public List<Integer> postorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<>();
    if(root == null) return list;
    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);
    while(!stack.empty()){
        root = stack.pop();
        list.add(0, root.val);
        if(root.left != null) stack.push(root.left);
        if(root.right != null) stack.push(root.right);
    }
    return list;
}

后续遍历:
public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<>();
    if(root == null) return list;
    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);
    while(!stack.empty()){
        root = stack.pop();
        list.add(root.val);
        if(root.right != null) stack.push(root.right);
        if(root.left != null) stack.push(root.left);
    }
    return list;
}

中序遍历:
public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<>();
    if(root == null) return list;
    Stack<TreeNode> stack = new Stack<>();
    while(root != null || !stack.empty()){
        while(root != null){
            stack.push(root);
            root = root.left;
        }
        root = stack.pop();
        list.add(root.val);
        root = root.right;
    }
    return list;
}