Java二叉树的四种遍历方式详解

public class TreeNode {
    public int data;
    public TreeNode leftChild;
    public TreeNode rightChild;
    public TreeNode(int data){
        this.data = data;
    }
}

/**
     * 构建二叉树
     * @param list   输入序列
     * @return
     */
    public static TreeNode createBinaryTree(LinkedList<Integer> list){
        TreeNode node = null;
        if(list == null || list.isEmpty()){
            return null;
        }
        Integer data = list.removeFirst();
        if(data!=null){
            node = new TreeNode(data);
            node.leftChild = createBinaryTree(list);
            node.rightChild = createBinaryTree(list);
        }
        return node;
    }

/**
     * 二叉树前序遍历   根-> 左-> 右
     * @param node    二叉树节点
     */
    public static void preOrderTraveral(TreeNode node){
        if(node == null){
            return;
        }
        System.out.print(node.data+" ");
        preOrderTraveral(node.leftChild);
        preOrderTraveral(node.rightChild);
    }

/**
     * 二叉树中序遍历   左-> 根-> 右
     * @param node   二叉树节点
     */
    public static void inOrderTraveral(TreeNode node){
        if(node == null){
            return;
        }
        inOrderTraveral(node.leftChild);
        System.out.print(node.data+" ");
        inOrderTraveral(node.rightChild);
    }

/**
     * 二叉树后序遍历   左-> 右-> 根
     * @param node    二叉树节点
     */
    public static void postOrderTraveral(TreeNode node){
        if(node == null){
            return;
        }
        postOrderTraveral(node.leftChild);
        postOrderTraveral(node.rightChild);
        System.out.print(node.data+" ");
    }

public static void preOrderTraveralWithStack(TreeNode node){
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode treeNode = node;
        while(treeNode!=null || !stack.isEmpty()){
            //迭代访问节点的左孩子，并入栈
            while(treeNode != null){
                System.out.print(treeNode.data+" ");
                stack.push(treeNode);
                treeNode = treeNode.leftChild;
            }
            //如果节点没有左孩子，则弹出栈顶节点，访问节点右孩子
            if(!stack.isEmpty()){
                treeNode = stack.pop();
                treeNode = treeNode.rightChild;
            }
        }
    }

public static void inOrderTraveralWithStack(TreeNode node){
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode treeNode = node;
        while(treeNode!=null || !stack.isEmpty()){
            while(treeNode != null){
                stack.push(treeNode);
                treeNode = treeNode.leftChild;
            }
            if(!stack.isEmpty()){
                treeNode = stack.pop();
                System.out.print(treeNode.data+" ");
                treeNode = treeNode.rightChild;
            }
        }
    }

public static void postOrderTraveralWithStack(TreeNode node){
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode treeNode = node;
        TreeNode lastVisit = null;   //标记每次遍历最后一次访问的节点
        while(treeNode!=null || !stack.isEmpty()){//节点不为空，结点入栈，并且指向下一个左孩子
            while(treeNode!=null){
                stack.push(treeNode);
                treeNode = treeNode.leftChild;
            }
            //栈不为空
            if(!stack.isEmpty()){
                //出栈
                treeNode = stack.pop();
                /**
                 * 这块就是判断treeNode是否有右孩子，
                 * 如果没有输出treeNode.data，让lastVisit指向treeNode，并让treeNode为空
                 * 如果有右孩子，将当前节点继续入栈，treeNode指向它的右孩子,继续重复循环
                 */
                if(treeNode.rightChild == null || treeNode.rightChild == lastVisit) {
                    System.out.print(treeNode.data + " ");
                    lastVisit = treeNode;
                    treeNode  = null;
                }else{
                    stack.push(treeNode);
                    treeNode = treeNode.rightChild;
                }
            }
        }
    }

public static void levelOrder(TreeNode root){
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()){
            root = queue.pop();
            System.out.print(root.data+" ");
            if(root.leftChild!=null) queue.add(root.leftChild);
            if(root.rightChild!=null) queue.add(root.rightChild);
        }
    }