剑指 Offer 32 - II. 从上到下打印二叉树 II

剑指 Offer 32 - II. 从上到下打印二叉树 II

还是比较常规的bfs搜索，先用队列添加每一层，在遍历每一层的时候将每个节点的左右子树分别按顺序加入队列中即可。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if(root == null) return ans;
        Queue<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        while(!q.isEmpty()) {
            int size = q.size();
            List<Integer> level = new ArrayList<>(size);
            for(int i = 0; i < size; i++) {
                var p = q.poll();
                level.add(p.val);
                if(p.left != null) q.offer(p.left);
                if(p.right != null) q.offer(p.right);
            }
            ans.add(level);
        }
        return ans;
    }
}