0103-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) {
        if (!root)
            return {};
        queue<TreeNode*> q;
        vector<vector<int>> res;
        q.push(root);
        int dir = 1; // dir 1: left, -1: right
        while (!q.empty()) {
            int size = q.size();
            vector<int> level;
            for (int i = 0; i < size; i++) {
                TreeNode* node = q.front();
                q.pop();
                if (node->left)
                    q.push(node->left);
                if (node->right)
                    q.push(node->right);
                level.push_back(node->val);
            }
            if (dir == -1) {
                reverse(level.begin(), level.end());
            }
            res.push_back(level);
            dir = (dir == 1) ? -1 : 1;
        }
        return res;
    }
};