Problem Statement

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).

**Example :**Given binary tree

`3

/ \ 9 20 / \ 15 7`

return

[ [3], [20, 9], [15, 7] ]

Problem Link

ZigZag Level Order Traversal BT - InterviewBit

Code (Using Deque)

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
vector<vector<int> > Solution::zigzagLevelOrder(TreeNode* root) {
    
    vector<vector<int>> res;
    
    if(!root)
    return res;
    
    deque<TreeNode*> dq;
    
    dq.push_front(root);
    int turn = 0;
    while(!dq.empty())
    {
        int lsize = dq.size();
        vector<int> temp;
        
        if(turn==0)
        {
            while(lsize--)
            {
            TreeNode *node = dq.front();
            dq.pop_front();
            temp.push_back(node->val);
            if(node->left)
            dq.push_back(node->left);
            
            if(node->right)
            dq.push_back(node->right);
            }
        }
        else
        {
            while(lsize--)
            {
            TreeNode *node = dq.back();
            dq.pop_back();
            temp.push_back(node->val);
            if(node->right)
            dq.push_front(node->right);
            
            if(node->left)
            dq.push_front(node->left);
            }
        }
        res.push_back(temp);
        turn = 1-turn;
    }
    return res;
}