leetcode二叉树相关模板

import java.util.*;
 
public class Main {
    public static void main(String[] args) {
 
        //Integer[] arr = {3, 9, 20, null, null, 15, 7};
        Integer[] arr = {1,2,3,4,null,5,null,null,null,null,null,null,6};
        TreeNode tree = createTree(arr, 0);
        System.out.println("层次：");
        printLevelOrder(tree);
        System.out.println();
        System.out.println("前序：");
        printPreOrder(tree);
        System.out.println();
        System.out.println("中序：");
        printMidOrder(tree);
        System.out.println();
        System.out.println("后序：");
        printAfterOrder(tree);
        System.out.println();
    }
 
    /**
     * 创建一个二叉树（通过层次法）
     * Integer[] arr = {1,2,3,4,null,5,null,null,null,null,null,null,6};
     * ---------1
     * ----2         3
     * -4         5
     * -------------6
     */
 
    public static TreeNode createTree(Integer[] array, int index) {
        TreeNode root = null;
        if (index < array.length) {
            Integer value = array[index];
            if (value == null) {
                return null;
            }
            root = new TreeNode(value);
            root.left = createTree(array, 2 * index + 1);
            root.right = createTree(array, 2 * index + 2);
        }
        return root;
 
    }
 
    /**
     * 层次遍历
     */
    public static void printLevelOrder(TreeNode root) {
        Queue queue = new ArrayDeque<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for(int i = 0; i < size; i++) {
                // 出队
                TreeNode node = queue.poll();
                System.out.print(node.val == null ? "" : node.val + " ");
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            System.out.println();
 
        }
    }
 
 
    /**
     * 前序遍历
     */
    public static void printPreOrder(TreeNode tree) {
        if (tree != null) {
            System.out.print(tree.val == null ? "" : tree.val + " ");
            printPreOrder(tree.left);
            printPreOrder(tree.right);
        }
    }
 
    /**
     * 中序遍历
     */
    public static void printMidOrder(TreeNode tree) {
        if (tree != null) {
            printMidOrder(tree.left);
            System.out.print(tree.val == null ? "" : tree.val + " ");
            printMidOrder(tree.right);
        }
    }
 
    /**
     * 后序遍历
     */
    public static void printAfterOrder(TreeNode tree) {
        if (tree != null) {
            printAfterOrder(tree.left);
            printAfterOrder(tree.right);
            System.out.print(tree.val == null ? "" : tree.val + " ");
        }
    }
 
}
 
class TreeNode {
    Integer val;
    TreeNode left;
    TreeNode right;
 
    TreeNode() {
    }
 
    TreeNode(Integer val) {
        this.val = val;
    }
 
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}