数据结构-树

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学习数据结构算法题

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public class TreeNodeDemo {
    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int val) {
            this.val = val;
        }
    }

    public static void main(String[] args) {
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        node1.left = node2;
        node1.right = node3;
        node2.left = node4;
        node2.right = node5;

        List<List<Integer>> ret = new ArrayList<>();

        afterOrderTreeStack(node1);
    }

    // 先序遍历
    private static void beforeOrderTree(TreeNode node) {
        if (node == null) return;
        System.out.println(node.val);
        beforeOrderTree(node.left);
        beforeOrderTree(node.right);
    }

    private static void beforeOrderTreeStack(TreeNode node) {
        if (node == null) return;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(node);
        while (!stack.isEmpty()) {
            TreeNode pop = stack.pop();
            System.out.println(pop.val);
            if (pop.right != null) stack.push(pop.right);
            if (pop.left != null) stack.push(pop.left);
        }
    }

    // 中序遍历
    private static void mobileOrderTree(TreeNode node) {
        if (node == null) return;
        beforeOrderTree(node.left);
        System.out.println(node.val);
        beforeOrderTree(node.right);
    }

    private static void mobileOrderTreeStack(TreeNode node) {
        if (node == null) return;
        Stack<TreeNode> stack = new Stack<>();
        while (node != null || !stack.isEmpty()) {
            while (node != null) {
                stack.push(node);
                node = node.left;
            }
            TreeNode pop = stack.pop();
            System.out.println(pop.val);
            node = pop.right;
        }
    }

    // 后序遍历
    private static void afterOrderTree(TreeNode node) {
        if (node == null) return;
        beforeOrderTree(node.left);
        System.out.println(node.val);
        beforeOrderTree(node.right);
    }

    private static void afterOrderTreeStack(TreeNode node) {
        if (node == null) return;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(node);
        while (!stack.isEmpty()) {
            TreeNode pop = stack.pop();
            System.out.println(pop.val);
            if (pop.left != null) stack.push(pop.left);
            if (pop.right != null) stack.push(pop.right);
        }
        // 输出需要进行反转
    }

    // 层级遍历-递归
    private static void levelOrderTreeRecursion(List<List<Integer>> ret, TreeNode node, int deep) {
        if (node == null) return;
        if (ret.size() == deep) ret.add(new ArrayList<>());
        ret.get(deep).add(node.val);
        levelOrderTreeRecursion(ret, node.left, deep + 1);
        levelOrderTreeRecursion(ret, node.right, deep + 1);
    }

    // 层级遍历-队列
    private static void levelOrderTreeQueue(TreeNode node) {
        if (node == null) {
            return;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(node);
        while (!queue.isEmpty()) {
            TreeNode poll = queue.poll();
            System.out.println(poll.val);
            if (poll.left != null) queue.add(poll.left);
            if (poll.right != null) queue.add(poll.right);
        }
    }
}