[leetcode]Binary Tree Zigzag Level Order Traversal

DFS,用一个变量来记录方向

从左往右就是push_back

从右往左就是push_front ,木有这个函数就用vec.insert(vec.begin() , val)，虽然效率不怎样

/**
 * 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);
    }
};