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103. Binary Tree Zigzag Level Order Traversal

MediumOpen on LeetCodeProblem statement

Problem Statement

103. Binary Tree Zigzag Level Order Traversal

Medium

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:

C++

Source file
/**
 * 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>> ans;
        if(root == NULL)
            return ans;
        queue<TreeNode*> info;
        info.push(root);
        bool flag = false;
        while(!info.empty()){
            int n = info.size();
            vector<int> inter;
            for(; n>0; n--){
                TreeNode* top = info.front();
                info.pop();
                if(top->left != NULL)
                    info.push(top->left);
                if(top->right != NULL)
                    info.push(top->right);
                inter.push_back(top->val);
            }
            if(flag)
                reverse(inter.begin(), inter.end());
            ans.push_back(inter);
            flag = !flag;
        }
        return ans;
    }
};