Binary Tree Zigzag Level Order Traversal


Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

 

Constraints:

  • The number of nodes in the tree is in the range [0, 2000].
  • -100 <= Node.val <= 100

Solution

/**
 * 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;
        if(root==NULL){
            return v;
        }
        stack<int>st;
        vector<int>temp;
        queue<TreeNode*>q;
        q.push(root);
        q.push(NULL);
        int i=1;
        while(!q.empty()){
            TreeNode* f=q.front();
            q.pop();
            if(f==NULL){
                if(i%2==0){
                    while(!st.empty()){
                        int k=st.top();
                        st.pop();
                        temp.push_back(k);
                    }
                }
                v.push_back(temp);
                temp.clear();
                i++;
                if(!q.empty()){
                    q.push(NULL);
                }
            }
            else{
                if(i%2==0){
                    st.push(f->val);
                }
                else{
                    temp.push_back(f->val);
                }
                if(f->left){
                    q.push(f->left);
                }
                if(f->right){
                    q.push(f->right);
                }
            }
        }
        return v;
    }
};