102. 二叉树的层序遍历

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.List;
import java.util.Queue;

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

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            List<Integer> curr = new ArrayList<>();
            //当前这层队列的长度
            int size = queue.size();
            //遍历当前层的节点
            for (int i = 0; i < size; i++) {
                TreeNode currNode = queue.poll();
                if (currNode.left != null) {
                    queue.add(currNode.left);
                }
                if (currNode.right != null) {
                    queue.add(currNode.right);
                }
                curr.add(currNode.val);
            }
            res.add(curr);

        }
        return res;
    }
}