二叉树的深度优先遍历和广度优先遍历

深度优先遍历：前序遍历，中序遍历，后序遍历
广度优先遍历：层次遍历

定义二叉树node节点：

public class TreeNode {
    private int data;
    private TreeNode right,left;

    public int getData() {
        return data;
    }

    public void setData(int data) {
        this.data = data;
    }

    public TreeNode getRight() {
        return right;
    }

    public void setRight(TreeNode right) {
        this.right = right;
    }

    public TreeNode getLeft() {
        return left;
    }

    public void setLeft(TreeNode left) {
        this.left = left;
    }

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

二叉树遍历方法：

public class TreeNodeDemo {

    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);
        TreeNode node6 = new TreeNode(6);
        TreeNode node7 = new TreeNode(7);
        TreeNode node8 = new TreeNode(8);
        node1.setLeft(node2);
        node1.setRight(node3);
        node2.setLeft(node4);
        node2.setRight(node5);
        node5.setLeft(node7);
        node5.setRight(node8);
        node3.setLeft(node6);

        preOrderRecur(node1);
        System.out.println();
        preOrderUnRecur(node1);
        System.out.println();
        inOrderRecur(node1);
        System.out.println();
        inOrderUnRecur(node1);
        System.out.println();
        posOrderRecur(node1);
        System.out.println();
        posOrderUnRecur(node1);
        System.out.println();
        levelTraverse(node1);
    }

    /**
     * 递归前序遍历 根-左-右
     *
     * @param head
     */
    public static void preOrderRecur(TreeNode head) {
        if (head == null) {
            return;
        }
        System.out.print(head.getData() + " ");
        preOrderRecur(head.getLeft());
        preOrderRecur(head.getRight());
    }

    /**
     * 非递归前序遍历 栈实现
     * 反着顺序，pop出栈先进后出
     * 1、先放中节点
     * 2、有右节点放右节点
     * 3、有左节点放左节点
     *
     * @param head
     */
    public static void preOrderUnRecur(TreeNode head) {
        if (head == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.add(head);
        while (!stack.isEmpty()) {
            TreeNode pop = stack.pop();
            System.out.print(pop.getData() + " ");
            if (pop.getRight() != null) {
                stack.add(pop.getRight());
            }
            if (pop.getLeft() != null) {
                stack.add(pop.getLeft());
            }
        }

    }

    /**
     * 递归中序遍历 左-根-右
     *
     * @param head
     */
    public static void inOrderRecur(TreeNode head) {
        if (head == null) {
            return;
        }
        inOrderRecur(head.getLeft());
        System.out.print(head.getData() + " ");
        inOrderRecur(head.getRight());
    }

    /**
     * 非递归中序遍历 栈实现
     * 1、左节点不为null则压入左节点
     * 2、左节点为null时，pop并打印左节点
     * 3、再往右，右节点为null时，pop并打印
     * 4、右节点不为null时，压入右节点
     *
     * @param head
     */
    public static void inOrderUnRecur(TreeNode head) {
        if (head == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || head != null) {
            if (head != null) {
                stack.add(head);
                head = head.getLeft();
            } else {
                head = stack.pop();
                System.out.print(head.getData() + " ");
                head = head.getRight();
            }
        }
    }

    /**
     * 递归后序遍历 左-右-根
     *
     * @param head
     */
    public static void posOrderRecur(TreeNode head) {
        if (head == null) {
            return;
        }
        posOrderRecur(head.getLeft());
        posOrderRecur(head.getRight());
        System.out.print(head.getData() + " ");
    }

    /**
     * 非递归后序遍历 栈实现
     * 和前序遍历一样的只不过是使用了两个栈
     * 1、在前序遍历的时候将 中 右 左 节点压栈
     * 2、在原来是打印的地方不打印，将本该打印的节点压到第二个栈中
     * 3、这样第二个栈的出栈顺序就是 右 左 中了
     *
     * @param head
     */
    public static void posOrderUnRecur(TreeNode head) {
        if (head == null) {
            return;
        }
        Stack<TreeNode> stack1 = new Stack<>();
        Stack<TreeNode> stack2 = new Stack<>();
        stack1.add(head);
        while (!stack1.isEmpty()) {
            head = stack1.pop();
            stack2.add(head);
            if (head.getLeft() != null) {
                stack1.add(head.getLeft());
            }
            if (head.getRight() != null) {
                stack1.add(head.getRight());
            }
        }
        while (!stack2.isEmpty()) {
            System.out.print(stack2.pop().getData() + " ");
        }
    }

    /**
     * 层序遍历 队列实现
     * 先进先出
     *
     * @param head
     */
    public static void levelTraverse(TreeNode head) {
        if (head == null) {
            return;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(head);
        while (!queue.isEmpty()) {
            TreeNode poll = queue.poll();
            System.out.print(poll.getData() + " ");
            if (poll.getLeft() != null) {
                queue.add(poll.getLeft());
            }
            if (poll.getRight() != null) {
                queue.add(poll.getRight());
            }
        }
    }
}