103. Binary Tree Zigzag Level Order Traversal

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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]
]

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution{
private:
    vector<vector<int>> vec;
    void bfs(TreeNode*root , int depth){
        if(!root)
            return;
        if(root)
        {
            if(vec.size() < depth + 1) {
                vector<int> tmp;
                tmp.push_back(root->val);
                vec.push_back(tmp);
            }
            else{
                if(depth % 2 == 0){
                    vec[depth].push_back(root->val);
                }
                else{
                    vec[depth].insert(vec[depth].begin(),root->val);
                }
            }
        }
        if(root->left){
            bfs(root->left, depth + 1);
        }
        if(root->right){
            bfs(root->right, depth+1);
        }

    }

public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        bfs(root, 0);
        return vec;
    }
};