【Leetcode】Binary Tree Zigzag Level Order Traversal

题目链接：https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/

题目：

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its zigzag level order traversal as:

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

思路：

层序遍历，再将遍历结果改为需要的顺序。。

算法：

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {      List<List<Integer>> lists = new ArrayList<List<Integer>>();      Queue<TreeNode> q = new LinkedList<TreeNode>();      if (root == null)          return lists;      q.offer(root);      while (!q.isEmpty()) {          List<Integer> list = new ArrayList<Integer>();          int size = q.size();          for (int i = 0; i < size; i++) { // 该层结点数              TreeNode t = q.poll();              list.add(t.val);              if (t.left != null) {                  q.offer(t.left);              }              if (t.right != null) {                  q.offer(t.right);              }          }          lists.add(list);      }      // 将层序结果改为zagzag顺序      boolean flag = true;      List<List<Integer>> res = new ArrayList<List<Integer>>();      for (List<Integer> ll : lists) {          if (flag) {              res.add(ll);          } else {              List<Integer> tmp = new ArrayList<Integer>();              for (int i = ll.size() - 1; i >= 0; i--) {                  tmp.add(ll.get(i));              }              res.add(tmp);          }          flag = !flag;      }      return res;  }