103.binary-tree-zigzag-level-order-traversal.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>> level = levelOrder(root);        
        for(int i=0; i<level.size(); i++){
            if(i%2){
                reverse(level[i].begin(), level[i].end());
            }
        }
        return level;
    }
    
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if(!root)
            return res;
    
            
        queue<TreeNode*> q;
        q.push(root);
        TreeNode* node;
        res.push_back({});
        q.push(NULL);
        while(true){
            node = q.front();
            q.pop();
            if(node)
            {
                res.back().push_back(node->val);
                if(node->left)
                    q.push(node->left);
                if(node->right)
                    q.push(node->right);
            }else{
                if(q.empty())
                    return res;
                q.push(NULL);
                res.push_back({});
            }
        }
        return res;
    }
};