Niuke-top101-bm26

The core idea ： Non recursive sequence traversal

import java.util.*;

/* * public class TreeNode { * int val = 0; * TreeNode left = null; * TreeNode right = null; * } */

public class Solution {
    
    /** * * @param root TreeNode class  * @return int integer ArrayList<ArrayList<>> */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
    
        // write code here
        ArrayList<ArrayList<Integer>> lists = new ArrayList<>();
        Deque<TreeNode> deque = new ArrayDeque<>();
        deque.addLast(root);
        while(!deque.isEmpty()){
    
            ArrayList<Integer> list = new ArrayList<>();
            int size = deque.size();
            for(int i = 0; i < size; 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);
        }
        return lists;
    }
}