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]
]

分析： 层序遍历的，注意Zigzag的顺序

/**
 * 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>> res;
        if(root == NULL)
            return res;
        queue<TreeNode* > nodesQue;
        bool flag = true;
        nodesQue.push(root);
        while(!nodesQue.empty()){
            int size = nodesQue.size();
            vector<int> temp(size,0);
            for(int i=0; i<size;i++){
                TreeNode* node = nodesQue.front();
                nodesQue.pop();
                
                int index = flag? i: size-1-i;
                temp[index] = node->val;
                if(node->left)
                    nodesQue.push(node->left);
                if(node->right)
                    nodesQue.push(node->right);
            }
            flag = !flag;
            res.push_back(temp);
        }
        
        return res;
    }
};