【剑指Offer学习】【面试题61：按之字形顺序打印二叉树】

题目：请实现一个函数按照之字形顺序打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右到左的顺序打印，即第一行按照从左到右的顺序打印，第二层按照从右到左顺序打印，第三行再按照从左到右的顺序打印，其他以此类推。

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解题思路

　　按之字形顺序打印二叉树需要两个栈。我们在打印某一行结点时，把下一层的子结点保存到相应的栈里。如果当前打印的是奇数层，则先保存左子结点再保存右子结点到一个栈里；如果当前打印的是偶数层，则先保存右子结点再保存左子结点到第二个栈里。

结点定义

private static class BinaryTreeNode {    private int val;    private BinaryTreeNode left;    private BinaryTreeNode right;    public BinaryTreeNode() {    }    public BinaryTreeNode(int val) {        this.val = val;    }    @Override    public String toString() {        return val + "";    }}

代码实现

import java.util.LinkedList;import java.util.List;public class Test61 {    private static class BinaryTreeNode {        private int val;        private BinaryTreeNode left;        private BinaryTreeNode right;        public BinaryTreeNode() {        }        public BinaryTreeNode(int val) {            this.val = val;        }        @Override        public String toString() {            return val + "";        }    }    public static void print(BinaryTreeNode root) {        if (root == null) {            return;        }        List<BinaryTreeNode> current = new LinkedList<>();        List<BinaryTreeNode> reverse = new LinkedList<>();        int flag = 0;        BinaryTreeNode node;        current.add(root);        while (current.size() > 0) {            // 从最后一个开始取            node = current.remove(current.size() - 1);            System.out.printf("%-3d", node.val);            // 当前是从左往右打印的，那就按从左往右入栈            if (flag == 0) {                if (node.left != null) {                    reverse.add(node.left);                }                if (node.right != null) {                    reverse.add(node.right);                }            }            // 当前是从右往左打印的，那就按从右往左入栈            else {                if (node.right != null) {                    reverse.add(node.right);                }                if (node.left != null) {                    reverse.add(node.left);                }            }            if (current.size() == 0) {                flag = 1 - flag;                List<BinaryTreeNode> tmp = current;                current = reverse;                reverse = tmp;                System.out.println();            }        }    }    public static void main(String[] args) {        BinaryTreeNode n1 = new BinaryTreeNode(1);        BinaryTreeNode n2 = new BinaryTreeNode(2);        BinaryTreeNode n3 = new BinaryTreeNode(3);        BinaryTreeNode n4 = new BinaryTreeNode(4);        BinaryTreeNode n5 = new BinaryTreeNode(5);        BinaryTreeNode n6 = new BinaryTreeNode(6);        BinaryTreeNode n7 = new BinaryTreeNode(7);        BinaryTreeNode n8 = new BinaryTreeNode(8);        BinaryTreeNode n9 = new BinaryTreeNode(9);        n1.left = n2;        n1.right = n3;        n2.left = n4;        n2.right = n5;        n3.left = n6;        n3.right = n7;        n4.left = n8;        n4.right = n9;        print(n1);    }}

运行结果

说明

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