102_二叉树的层序遍历.md

import java.util.*;


/**
 * @author : xfhy
 * Create time : 2020年8月13日08:29:35
 * Description : 102. 二叉树的层序遍历
 * source : https://leetcode-cn.com/problems/binary-tree-level-order-traversal/
 */
public class Solution {

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

        TreeNode(int x) {
            val = x;
        }
    }

    /**
     * 思路:  使用一个变量来记录每一层的节点个数
     */
    public static List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> lists = new LinkedList<>();
        if (root == null) {
            return lists;
        }
        Deque<TreeNode> deque = new LinkedList<>();
        deque.push(root);
        while (!deque.isEmpty()) {
            int size = deque.size();
            List<Integer> levels = new LinkedList<>();
            for (int i = 0; i < size; i++) {
                //每次都取第一个
                TreeNode pop = deque.pollFirst();
                levels.add(pop.val);
                if (pop.left != null) {
                    deque.addLast(pop.left);
                }
                if (pop.right != null) {
                    deque.addLast(pop.right);
                }
            }
            lists.add(levels);
        }
        return lists;
    }

    public static TreeNode createBinaryTree(LinkedList<Integer> inputList) {
        if (inputList == null || inputList.isEmpty()) {
            return null;
        }
        TreeNode node = null;
        Integer data = inputList.removeFirst();
        if (data != null) {
            node = new TreeNode(data);
            node.left = createBinaryTree(inputList);
            node.right = createBinaryTree(inputList);
        }
        return node;
    }

    public static void main(String[] args) {
        /*LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(5, 4, 11, 7, null, null, 2, null, null, null, 8, 13, null, null, 4,
                5, null, null, 1));*/
        /*LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(3, 2, 9, null, null, 10, null,
                null, 8, null, 4));*/
        LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(3, 9, null, null, 20, 15, null, null, 7));
        LinkedList<Integer> integers2 = new LinkedList<>(Arrays.asList(5, 3, -2147483648, null, null, 2, null, null, 9));
        TreeNode binaryTree = createBinaryTree(integers);

        System.out.println(levelOrder(binaryTree));
    }

}