树：广度优先遍历1

package club.interview.tree;

import club.interview.tree.base.TreeNode;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/**
 * 从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。
 * fixme https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof/
 * <p>
 * 3
 * / 
 * 9  20
 * /  
 * 15   7
 * <p>
 * [3 9 20 15 7]
 * <p>
 * 知识点扫盲
 * 1. LinkedList是Queue的实现类
 * 2. 出队入队的操作分别是 poll，offer
 *
 * @author QuCheng on 2020/6/3.
 */
public class BFSTree {

    private TreeNode buildNode() {
        TreeNode t1 = new TreeNode(15);
        TreeNode t2 = new TreeNode(7);
        TreeNode t3 = new TreeNode(20, t1, t2);
        TreeNode t4 = new TreeNode(9);
        return new TreeNode(3, t4, t3);
    }

    public static void main(String[] args) {
        BFSTree b = new BFSTree();
        System.out.println(b.levelOrder(b.buildNode()));
    }

    /**
     * 列用队列FIFO的特性，遍历左右节点放入队列中，然后按层输出
     */
    public List<Integer> levelOrder(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue<TreeNode> temp = new LinkedList<>();
        temp.offer(root);
        while (!temp.isEmpty()) {
            TreeNode treeNode = temp.poll();
            result.add(treeNode.val);
            if (treeNode.left != null) {
                temp.offer(treeNode.left);
            }
            if (treeNode.right != null) {
                temp.offer(treeNode.right);
            }
        }
        return result;
    }
}