[leetcode]Binary Tree Zigzag Level Order Traversal

/**
* Definition for binary tree
* 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> > ans;
if(root == nullptr) return ans;
dfs(ans , root , 0 , true);
return ans;
}
private:
void dfs(vector<vector<int> >&ans , TreeNode* root , int dep , bool left) {
if(root == nullptr) return;
if(dep + 1 > ans.size()) ans.push_back(vector<int>());
if(left) ans[dep].push_back(root->val);
else ans[dep].insert(ans[dep].begin() , root->val);
dfs(ans , root->left,dep+1,!left);
dfs(ans , root->right,dep+1,!left);
}
};