根据层序遍历构造二叉树

层序遍历： [10,5,15,3,7,13,18,1,null,6]

//层序遍历的数组构造 二叉树
static TreeNode buildTree(Object[] result, int index) {
	if (result[index] == null) {
		return null;
	}
    // 根结点 root 初始化
	TreeNode root = new TreeNode((Integer)result[index]);
	root.left = null;
	root.right = null;

	int left = 2 * index + 1;
	int right = 2 * index + 2;

	//构造左子树
	if (left > result.length - 1) {
		root.left = null;
	} else {
		root.left = buildTree(result, left);
	}
       //构造右子树
	if (right > result.length - 1) {
		root.right = null;
	} else {
		root.right = buildTree(result, right);
	}
	return root;
}

package com.example.demo;

import java.util.LinkedList;

public class Main {

	static class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

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

		TreeNode() {
		}
	}

	//层序遍历的数组构造 二叉树
	static TreeNode buildTree(Object[] result, int index) {
		if (result[index] == null) {
			return null;
		}

		TreeNode root = new TreeNode((Integer)result[index]);
		root.left = null;
		root.right = null;

		int left = 2 * index + 1;
		int right = 2 * index + 2;

		if (left > result.length - 1) {
			root.left = null;
		} else {
			root.left = buildTree(result, left);

		}

		if (right > result.length - 1) {
			root.right = null;
		} else {
			root.right = buildTree(result, right);
		}
		return root;
	}

	//先序遍历 打印
	static void preOrderTraverse(TreeNode root) {
		//已经完成了对n件物品的选择（递归边界--死胡同）
		if (root == null) {
			return;
		}
		System.out.print(root.val + " ");
		preOrderTraverse(root.left);
		preOrderTraverse(root.right);
	}

	//层序遍历 打印
	static void levelOrderTraverse(TreeNode root) {
		LinkedList<TreeNode> queue = new LinkedList<>();
		if (root == null) {
			return;
		}
		queue.addLast(root);
		while (!queue.isEmpty()) {
			TreeNode temp = queue.getFirst();
			System.out.print(temp.val + " ");
			queue.removeFirst();

			if (temp.left != null) {
				queue.addLast(temp.left);
			}
			if (temp.right != null) {
				queue.addLast(temp.right);
			}
		}

	}

	public static void main(String[] args) {
		Object[] result1 = {10, 5, 15, 3, 7, 13, 18, 1, null, 6};
		preOrderTraverse(buildTree(result1, 0));
		System.out.println();
		levelOrderTraverse(buildTree(result1, 0));
		System.out.println();
		
	}

}

参考文章