103 二叉树的锯齿形层序遍历

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xv7pir/

cost: 8m34s

/**
 * 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 == nullptr) {
            return {};
        }
        queue<TreeNode*> q;
        q.push(root);
        vector<vector<int>> res;
        bool asc = true;
        while (!q.empty()) {
            int n = q.size();
            vector<int> vec(n);
            for (int i = 0; i < n; i++) {
                auto node = q.front();
                q.pop();
                if (asc) {
                    vec[i] = node->val;
                } else {
                    vec[n - 1 - i] = node->val;
                }
                if (node->left) {
                    q.push(node->left);
                }
                if (node->right) {
                    q.push(node->right);
                }
            }
            res.push_back(vec);
            asc = !asc;
        }
        return res;
    }
};