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]

]

class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > ans;
        vector<int> layer_v;
        if (!root){
            return ans;
        }
        
        int layer = 0;
        queue<pair<TreeNode*, int> > qu;        
        qu.push(make_pair(root,0));
        pair<TreeNode*, int> node;
        
        while(!qu.empty()){
            node = qu.front();
            qu.pop();
            if (node.second == layer){
                layer_v.push_back(node.first->val);
            }else{
                if (layer % 2){
                    reverse(layer_v.begin(),layer_v.end());
                }
                ans.push_back(layer_v);
                layer++;
                layer_v.clear();
                layer_v.push_back(node.first->val);
            }
            
            if (node.first->left){
                qu.push(make_pair(node.first->left,node.second+1));
            }
            if (node.first->right){
                qu.push(make_pair(node.first->right,node.second+1));
            }
        }
        if (layer % 2){
            reverse(layer_v.begin(),layer_v.end());
        }
        ans.push_back(layer_v);
        return ans;
    }
};