leetcode 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]
]

类似图的层级遍历，一定用BFS

/** * 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>> ret;        queue<TreeNode *> que;        if (root)            que.push(root);                while (!que.empty())        {            int size = que.size();            vector<int> temp;            for (int i = 0; i < size; i++)            {                TreeNode * p = que.front();                que.pop();                temp.push_back(p->val);                if (p->left)                    que.push(p->left);                if (p->right)                    que.push(p->right);            }            ret.push_back(temp);        }        for (int i = 1; i < ret.size(); i = i + 2)            reverse(ret[i].begin(), ret[i].end());        return ret;         }};