二叉树遍历的非递归写法-创新互联

这个相对来说比较简单，就是一层一层遍历，核心点是采用队列，queue

直接上代码

public static void levelTraversal(TreeNode root) {
        if (root == null) {
            return;
        }
        Queuequeue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode poll = queue.poll();
            System.out.println(poll.value);
            if (poll.left != null) {
                queue.offer(poll.left);
            }
            if (poll.right != null) {
                queue.offer(poll.right);
            }
        }
    }

而深度优先遍历，核心点是采用栈

直接上代码，投机取巧的写法

public static void preorderTraversal(TreeNode root) {
        if (root == null) {
            return;
        }
        Stackstack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode pop = stack.pop();
            System.out.println(pop.value);
            if (pop.right != null) {
                stack.push(pop.right);
            }
            if (pop.left != null) {
                stack.push(pop.left);
            }
        }
    }

标准写法

public static void preorderTraversal02(TreeNode root) {
        if (root == null) {
            return;
        }
        Stackstack = new Stack<>();
        TreeNode temp = root;
        while (temp != null || !stack.isEmpty()) {
            while (temp != null) {
                stack.push(temp);
                System.out.println(temp.value);
                temp = temp.left;
            }
            if (!stack.isEmpty()) {
                TreeNode pop = stack.pop();
                temp = pop.right;
            }
        }
    }

然后是中序遍历，中序遍历没有投机取巧的方法，只有一种标准写法

public static void middleorderTraversal(TreeNode root) {
        if (root == null) {
            return;
        }
        Stackstack = new Stack<>();
        TreeNode temp = root;
        while (temp != null || !stack.isEmpty()) {
            while (temp != null) {
                stack.push(temp);
                temp = temp.left;
            }
            if (!stack.isEmpty()) {
                TreeNode pop = stack.pop();
                System.out.println(pop.value);
                temp = pop.right;
            }
        }
    }

后序遍历，也有两种写法，一种投机取巧，一种标准写法

投机取巧的方式，代码如下

public static void postorderTraversal(TreeNode root) {
        if (root == null) {
            return;
        }
        Stackstack = new Stack<>();
        stack.push(root);
        Listlist = new LinkedList<>();
        while (!stack.isEmpty()) {
            TreeNode pop = stack.pop();
            list.add(pop);
            if (pop.left != null) {
                stack.push(pop.left);
            }
            if (pop.right != null) {
                stack.push(pop.right);
            }
        }
        Collections.reverse(list);
        list.stream().forEach(System.out::println);
    }

标准写法

public static void postorderTraversal02(TreeNode root) {
        if (root == null) {
            return;
        }
        Stackstack = new Stack<>();
        TreeNode temp = root, lastNode = null;
        while (temp != null || !stack.isEmpty()) {
            while (temp != null) {
                stack.push(temp);
                temp = temp.left;
            }
            // 查看当前栈顶元素
            temp = stack.peek();
            // 如果其右子树也为空，或者右子树已经访问过，则可以直接输出当前节点的值
            if (temp.right == null || temp.right == lastNode) {
                System.out.println(temp.value);
                stack.pop();
                // 把输出的节点赋值给lastNode游标，作为下次比对的依据
                lastNode = temp;
                temp = null;
            } else {
                // 否则，继续遍历右子树
                temp = temp.right;
            }
        }
    }

总结：

广度优先遍历比较简单，深度优先遍历，先序与中序标准写法是相同的代码结构，只需要一个指针，后续则不一样，需要记录上一次的遍历节点，需要两个指针，总的来说，练习一下都比较简单。

你是否还在寻找稳定的海外服务器提供商？创新互联www.cdcxhl.cn海外机房具备T级流量清洗系统配攻击溯源，准确流量调度确保服务器高可用性，企业级服务器适合批量采购，新人活动首月15元起，快前往官网查看详情吧