103. Binary Tree Zigzag Level Order Traversal

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,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its zigzag level order traversal as:

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

.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int> >w;
        if (root == nullptr) return w;
        queue<TreeNode *> q;
        q.push(root);
        while (!q.empty()) {
            int n = q.size();
            vector<int> v;
            for (int i = 0; i < n; ++i) {
                TreeNode *u = q.front();q.pop();
                if (u == nullptr) continue;
                q.push(u->left);
                q.push(u->right);
                v.push_back(u->val);
            }
            if (w.size()%2 == 1){
                reverse(v.begin(), v.end());
            }
            if (v.size())w.push_back(v);
        }
        return w;
    }
};