【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;  
}