Leetcode103. 二叉树的锯齿形层序遍历

题目传送：​​https://leetcode.cn/problems/binary-tree-zigzag-level-order-traversal/​​

运行效率

代码如下

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    // 处理边界情况
    if (root == null) {
      return result;
    }
    //层序遍历一般通过队列来实现
    ArrayDeque<TreeNode> arrayDeque = new ArrayDeque<>();
    arrayDeque.add(root);
    // -1代表从左往右，1代表从右往左
    int direction = -1; 
    while (arrayDeque.size() > 0) {
      // 用于存储本层遍历的结果
      ArrayList<Integer> list = new ArrayList<>();
      // 存储下一层要遍历的节点
      ArrayList<TreeNode> list1 = new ArrayList<>();
      while (arrayDeque.size() > 0) {
        // 弹出队列头部的元素
        TreeNode poll = arrayDeque.poll();
        list.add(poll.val);
        // 1代表从右往左
        if (direction == 1) {
          if (poll.right != null) {
            list1.add(poll.right);
          }
          if (poll.left != null) {
            list1.add(poll.left);
          }
        } else { // 0代表从左往右
          if (poll.left != null) {
            list1.add(poll.left);
          }
          if (poll.right != null) {
            list1.add(poll.right);
          }
        }
      }
      addQueue(list1,arrayDeque);
      direction = -direction;
      result.add(list);
    }
    return result;
  }

  public void addQueue(List<TreeNode> list,ArrayDeque<TreeNode> queue) {
    for(TreeNode treeNode:list){
      queue.addFirst(treeNode);
    }
  }