BinaryTreeZIgzagLevelOrderTraversal.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector <vector<int>> v;
        vector <int> u;
        if(!root)
        return v;
        stack <TreeNode*> s1, s2;
        s1.push(root);
        while(!s1.empty()||!s2.empty())
        {
            while(!s1.empty())
            {
                TreeNode *temp=s1.top();
                s1.pop();
                u.push_back(temp->val);
                if(temp->left)s2.push(temp->left);
                if(temp->right)s2.push(temp->right);
            }
            if(u.size()!=0)
            v.push_back(u);
            u.erase(u.begin(),u.end());
            while(!s2.empty())
            {
                TreeNode *temp=s2.top();
                s2.pop();
                u.push_back(temp->val);
                if(temp->right)s1.push(temp->right);
                if(temp->left)s1.push(temp->left);
            }
            if(u.size()!=0)
            v.push_back(u);
            u.erase(u.begin(),u.end());
        }
        return v; 
    }
};