Day06_剑指offer

Day06_剑指Offer

package com.sorrymaker.day3006;

import javax.swing.tree.TreeNode;
import java.util.ArrayList;
import java.util.LinkedList;
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
 * @Author nextGame
 * @Date 2021/8/17 19:58
 * @Version 1.0
 */
public class LevelOrder {

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

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

    public int[] levelOrder(TreeNode root) {
        if(root==null){
            return new int[0];
        }
        Queue<TreeNode> queue = new LinkedList<>();
        ArrayList<Integer> list = new ArrayList<>();
        queue.add(root);
        while (!queue.isEmpty()){
            TreeNode node = queue.poll();
            list.add(node.val);
            if(node.left!=null){
                queue.add(node.left);
            }
            if(node.right!=null){
                queue.add(node.right);
            }
        }
        int[] res = new int[list.size()];
        for (int i = 0; i < res.length; i++) {
            res[i] = list.get(i);
        }
        return res;
    }
}

package com.sorrymaker.day3006;

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

/**
 * 按每层输出。
 * @Author nextGame
 * @Date 2021/8/17 20:10
 * @Version 1.0
 */
public class LevelOrder2 {

    public class TreeNode {
        int val;
        LevelOrder2.TreeNode left;
        LevelOrder2.TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }
    public List<List<Integer>> levelOrder(TreeNode root) {

        Queue<TreeNode> queue =  new LinkedList<>();
        List<List<Integer>> list = new ArrayList<>();
        if(root!=null){
            queue.add(root);
        }
        while (!queue.isEmpty()){
            List<Integer> tmp = new ArrayList<>();

            for (int i = queue.size(); i > 0; i--) {
                TreeNode node = queue.poll();
                tmp.add(node.val);
                if(node.left!=null){
                    queue.add(node.left);
                }
                if(node.right!=null){
                    queue.add(node.right);
                }
            }
            list.add(tmp);
        }
        return list;
    }
}

package com.sorrymaker.day3006;

import java.util.*;

/**
 * @Author nextGame
 * @Date 2021/8/17 21:06
 * @Version 1.0
 */
public class LevelOrder3 {

    public class TreeNode {
        int val;
        LevelOrder3.TreeNode left;
        LevelOrder3.TreeNode right;

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


    public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> res = new ArrayList<>();
        if (root != null) {
            queue.add(root);
        }
        while (!queue.isEmpty()) {
            LinkedList<Integer> tmp = new LinkedList<>();
            for (int i = queue.size(); i > 0; i--) {
                TreeNode node = queue.poll();
                if (res.size() % 2 == 0) {
                    // 偶数层 -> 队列头部
                    tmp.addLast(node.val);
                } else {
                    // 奇数层 -> 队列尾部
                    tmp.addFirst(node.val);
                }
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            res.add(tmp);
        }
        return res;
    }
}