按之字形順序打印二叉樹

題目：請實現(xiàn)一個函數(shù)按照之字形順序打印二叉樹，即第一行按照從左到右的順序打印，第二層按照從右到左的順序打印，即第一行按照從左到右的順序打印，第二層按照從右到左順序打印，第三行再按照從左到右的順序打印，其他以此類推。

解題思路

按之字形順序打印二叉樹需要兩個棧。我們在打印某一行結(jié)點時，把下一層的子結(jié)點保存到相應(yīng)的棧里。如果當(dāng)前打印的是奇數(shù)層，則先保存左子結(jié)點再保存右子結(jié)點到一個棧里；如果當(dāng)前打印的是偶數(shù)層，則先保存右子結(jié)點再保存左子結(jié)點到第二個棧里。

結(jié)點定義

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 + "";
    }
}

代碼實現(xiàn)

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) {
            // 從最后一個開始取
            node = current.remove(current.size() - 1);
            System.out.printf("%-3d", node.val);
            // 當(dāng)前是從左往右打印的，那就按從左往右入棧
            if (flag == 0) {
                if (node.left != null) {
                    reverse.add(node.left);
                }
                if (node.right != null) {
                    reverse.add(node.right);
                }
            }
            // 當(dāng)前是從右往左打印的，那就按從右往左入棧
            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);
    }
}

運行結(jié)果

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