[算法] - 树的遍历以及优美打印

话不多说，直接上代码

import java.util.LinkedList;
import java.util.Queue;

public class BaseTree {

    public static void main(String[] args) {
        TreeNode treeNode = buildTreeNode(2);
        TreeOperation.beautifulShow(treeNode);
        System.out.println("<==============深度优先遍历==============>");
        deepShow(treeNode);
        System.out.println("<==============广度优先便利==============>");
        spanShow(treeNode);
    }

    protected static class TreeNode {
        int val = 0;
        TreeNode left = null;
        TreeNode right = null;

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

    protected static TreeNode buildTreeNode(int level){
        int i = 1;
        TreeNode treeNode = new TreeNode(i++);
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(treeNode);
        int size = 1;
        while (level-- > 0) {
            size *= 2;
        }

        while (--size > 0) {
            TreeNode poll = queue.poll();
            poll.left = new TreeNode(i++);
            poll.right = new TreeNode(i++);
            queue.offer(poll.left);
            queue.offer(poll.right);
        }

        return treeNode;
    }

    protected static void deepShow(TreeNode treeNode) {
        StringBuffer a1 = new StringBuffer();
        StringBuffer a2 = new StringBuffer();
        StringBuffer a3 = new StringBuffer();
        show(treeNode, a1, a2, a3);
        System.out.println("前序遍历: " + a1);
        System.out.println("中序遍历: " + a2);
        System.out.println("后序遍历: " + a3);
    }

    private static void show(TreeNode treeNode, StringBuffer a1, StringBuffer a2, StringBuffer a3){
        if (treeNode == null) {
            return;
        }
        a1.append(treeNode.val).append(" ");
        show(treeNode.left, a1, a2, a3);
        a2.append(treeNode.val).append(" ");
        show(treeNode.right, a1, a2, a3);
        a3.append(treeNode.val).append(" ");
    }

    private static void spanShow(TreeNode treeNode){
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(treeNode);
        System.out.print("广度遍历: ");
        while (!queue.isEmpty()) {
            TreeNode poll = queue.poll();
            if (poll == null) {
                continue;
            }
            System.out.print(poll.val + " ");
            queue.offer(poll.left);
            queue.offer(poll.right);
        }
    }



    public static class TreeOperation {
    /*
    树的结构示例：
              1
            /   \
          2       3
         / \     / \
        4   5   6   7
    */

        // 用于获得树的层数
        public static int getTreeDepth(TreeNode root) {
            return root == null ? 0 : (1 + Math.max(getTreeDepth(root.left), getTreeDepth(root.right)));
        }


        private static void writeArray(TreeNode currNode, int rowIndex, int columnIndex, String[][] res, int treeDepth) {
            // 保证输入的树不为空
            if (currNode == null) {return;}
            // 先将当前节点保存到二维数组中
            res[rowIndex][columnIndex] = String.valueOf(currNode.val);

            // 计算当前位于树的第几层
            int currLevel = ((rowIndex + 1) / 2);
            // 若到了最后一层，则返回
            if (currLevel == treeDepth) {return;}
            // 计算当前行到下一行，每个元素之间的间隔（下一行的列索引与当前元素的列索引之间的间隔）
            int gap = treeDepth - currLevel - 1;

            // 对左儿子进行判断，若有左儿子，则记录相应的"/"与左儿子的值
            if (currNode.left != null) {
                res[rowIndex + 1][columnIndex - gap] = "/";
                writeArray(currNode.left, rowIndex + 2, columnIndex - gap * 2, res, treeDepth);
            }

            // 对右儿子进行判断，若有右儿子，则记录相应的"\"与右儿子的值
            if (currNode.right != null) {
                res[rowIndex + 1][columnIndex + gap] = "\\";
                writeArray(currNode.right, rowIndex + 2, columnIndex + gap * 2, res, treeDepth);
            }
        }


        public static void beautifulShow(TreeNode root) {
            if (root == null) {System.out.println("EMPTY!");}
            // 得到树的深度
            int treeDepth = getTreeDepth(root);

            // 最后一行的宽度为2的（n - 1）次方乘3，再加1
            // 作为整个二维数组的宽度
            int arrayHeight = treeDepth * 2 - 1;
            int arrayWidth = (2 << (treeDepth - 2)) * 3 + 1;
            // 用一个字符串数组来存储每个位置应显示的元素
            String[][] res = new String[arrayHeight][arrayWidth];
            // 对数组进行初始化，默认为一个空格
            for (int i = 0; i < arrayHeight; i ++) {
                for (int j = 0; j < arrayWidth; j ++) {
                    res[i][j] = " ";
                }
            }

            // 从根节点开始，递归处理整个树
            // res[0][(arrayWidth + 1)/ 2] = (char)(root.val + '0');
            writeArray(root, 0, arrayWidth/ 2, res, treeDepth);

            // 此时，已经将所有需要显示的元素储存到了二维数组中，将其拼接并打印即可
            for (String[] line: res) {
                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < line.length; i ++) {
                    sb.append(line[i]);
                    if (line[i].length() > 1 && i <= line.length - 1) {
                        i += line[i].length() > 4 ? 2: line[i].length() - 1;
                    }
                }
                System.out.println(sb.toString());
            }
        }
    }
}

执行结果

      1      
    /   \    
  2       3  
 / \     / \ 
4   5   6   7
<==============深度优先遍历==============>
前序遍历: 1 2 4 5 3 6 7 
中序遍历: 4 2 5 1 6 3 7 
后序遍历: 4 5 2 6 7 3 1 
<==============广度优先便利==============>
广度遍历: 1 2 3 4 5 6 7