剑指 Offer 32 - III. 从上到下打印二叉树 III

剑指 Offer 32 - III. 从上到下打印二叉树 III

依旧是bfs搜索，这里可以添加一个boolean标识来代表从头部添加还是尾部添加。

/**
 * 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<>();
        boolean ordered = true;
        q.offer(root);
        while(!q.isEmpty()) {
            int size = q.size();
            Deque<Integer> level = new ArrayDeque<>();
            for(int i = 0; i < size; i++) {
                var p = q.poll();
                if(ordered) level.offerLast(p.val);
                else level.offerFirst(p.val);
                if(null != p.left) q.offer(p.left);
                if(null != p.right) q.offer(p.right);
            }
            List<Integer> list = new ArrayList(level);
            ans.add(list);
            ordered = !ordered;
        }
        return ans;
    }
}