LeetCode57|二叉树的锯齿形层次遍历

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回锯齿形层次遍历如下：

[
  [3],
  [20,9],
  [15,7]
]

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

public class ZigzagLevelOrderTest {
    public static void main(String[] args) {
        TreeNode t1 = new TreeNode(3);
        TreeNode t2 = new TreeNode(9);
        TreeNode t3 = new TreeNode(20);
        TreeNode t4 = new TreeNode(15);
        TreeNode t5 = new TreeNode(7);
        t1.left = t2;
        t1.right = t3;
        t3.left = t4;
        t3.right = t5;
        List<List<Integer>> listList = zigzagLevelOrder(t1);
        System.out.println("listList = " + listList);
    }

    public static List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> listList = new ArrayList<>();
        if (root == null) {
            return listList;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        boolean flag = true;
        queue.add(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            if (flag) {
                while (size-- > 0) {
                    TreeNode node = queue.pollFirst();
                    list.add(node.val);
                    if (node.left != null) {
                        queue.offer(node.left);
                    }
                    if (node.right != null) {
                        queue.offer(node.right);
                    }
                }

            } else {
                while (size-- > 0) {
                    TreeNode node = queue.pollLast();
                    list.add(node.val);
                    if (node.right != null) {
                        queue.push(node.right);
                    }
                    if (node.left != null) {
                        queue.push(node.left);
                    }
                }
            }
            flag = !flag;
            listList.add(list);
        }
        return listList;
    }
}