Solution103.java

package leetcode.tree.levelorder;

import leetcode.TreeNode;

import java.util.ArrayDeque;
import java.util.LinkedList;
import java.util.List;

public class Solution103 {

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();

        ArrayDeque<TreeNode> queue = new ArrayDeque<>();
        if (root != null) {
            queue.offer(root);
        }
        while (!queue.isEmpty()) {
            LinkedList<Integer> level = new LinkedList<>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (res.size() % 2 == 0) {
                    level.addLast(node.val);
                } else {
                    level.addFirst(node.val);
                }
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            res.add(level);
        }

        return res;
    }
}