二叉树的遍历(java实现)

二叉树的实现

    public static class Node {
        private int val;
        private Node left;
        private Node right;

        public Node(int val, Node left, Node right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }

前序遍历

递归实现

    public static void preOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        System.out.print(head.val + " ");
        preOrderRecur(head.left);
        preOrderRecur(head.right);
    }

非递归实现

public static void preOrderUnRecur(Node head) {
        if (head == null) {
            return;
        }
        Stack<Node> stack = new Stack<>();
        stack.push(head);
        while (!stack.isEmpty()) {
            head = stack.pop();
            System.out.print(head.val + " ");
            if (head.right != null) {
                stack.push(head.right);
            }
            if (head.left != null) {
                stack.push(head.left);
            }
        }
    }

中序遍历

递归实现

    public static void inOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        inOrderRecur(head.left);
        System.out.print(head.val + " ");
        inOrderRecur(head.right);
    }

非递归实现

    public static void inOrderUnRecur(Node head) {
        if (head != null) {
            Stack<Node> stack = new Stack<>();
            while (!stack.isEmpty() || head != null) {
                if (head != null) {
                    stack.push(head);
                    head = head.left;
                } else {
                    head = stack.pop();
                    System.out.print(head.val + " ");
                    head = head.right;
                }
            }
        }
    }

后序遍历

递归实现

    public static void posOrderRecur(Node head){
        if(head == null){
            return;
        }
        posOrderRecur(head.left);
        posOrderRecur(head.right);
        System.out.print(head.val + " ");
    }

非递归实现

    public static void posOrderUnRecur(Node head) {
        if (head == null) {
            return;
        }
        Stack<Node> stack1 = new Stack<>();
        Stack<Node> stack2 = new Stack<>();
        stack1.push(head);
        while (!stack1.isEmpty()) {
            head = stack1.pop();
            stack2.push(head);
            if (head.left != null) {
                stack1.push(head.left);
            }
            if (head.right != null) {
                stack1.push(head.right);
            }
        }
        while (!stack2.isEmpty()) {
            System.out.print(stack2.pop().val + " ");
        }
    }