牛客-TOP101-BM27

核心思路：层序遍历+Deque

import java.util.*;
import java.util.ArrayList;

/* public class TreeNode { int val = 0; TreeNode left = null; TreeNode right = null; public TreeNode(int val) { this.val = val; } } */
public class Solution {
    
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
    
        Deque<TreeNode> deque = new ArrayDeque<>();
        ArrayList<ArrayList<Integer>> lists = new ArrayList<>();
        if(pRoot != null){
    
            deque.addLast(pRoot);
        }
        while(!deque.isEmpty()){
    
            int count = deque.size();
            ArrayList<Integer> list = new ArrayList<>();
            for(int i = 0; i < count; i++){
    
                TreeNode t = deque.pollFirst();
                list.add(t.val);
                if(t.left != null){
    
                    deque.addLast(t.left);
                }if(t.right != null){
    
                    deque.addLast(t.right);
                }
            }
            lists.add(list);
            if(deque.isEmpty()){
    
                break;
            }
            count = deque.size();
            ArrayList<Integer> list1 = new ArrayList<>();
            for(int i = 0; i < count; i++){
    
                TreeNode t = deque.pollLast();
                list1.add(t.val);
                if(t.right != null){
    
                    deque.addFirst(t.right);
                }if(t.left != null){
    
                    deque.addFirst(t.left);
                }
            }
            lists.add(list1);
            
        }
        return lists;
    }

}

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