[LeetCode]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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

标准BFS，层序遍历。

/**
 * 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;
        vector<int> traversal;
        int level = 0;
        queue<TreeNode*> que;
        if(root==NULL)  return res;
        que.push(root);
        while(!que.empty()){
            int len = que.size();
            traversal.clear();
            for(int i=0; i<len; ++i){//BFS搜索每次把一整层的内容出队列
                TreeNode* FNode = que.front();
                traversal.push_back(FNode->val);
                if(FNode->left) que.push(FNode->left);
                if(FNode->right) que.push(FNode->right);
                que.pop();
            }
            if(level%2==0){
                res.push_back(traversal);
            }
            if(level%2==1){
                res.push_back(vector<int>(traversal.rbegin(),traversal.rend()));//翻转vector数组
            }
            level++;
        }
        return res;
    }
};