剑指Offer_22

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题目

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

解题思路

层序遍历二叉树

将ArrayList换成LinkedList效率会高一点……

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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<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> ret = new ArrayList<>();
        ArrayList<TreeNode> nodeList = new ArrayList<>();
        if(root == null){
            return ret;
        }
        nodeList.add(root);
        while(!nodeList.isEmpty()){
            TreeNode node = nodeList.get(0);
            nodeList.remove(0);
            ret.add(node.val);
            if(node.left != null){
                nodeList.add(node.left);
            }
            if(node.right != null){
                nodeList.add(node.right);
            }
        }
        return ret;
    }
}