102 二叉树的层序遍历

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xnldjj/

https://leetcode.cn/problems/binary-tree-level-order-traversal/description/

cost: 6m56s

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

• 时间复杂度：O(n)
• 空间复杂度：O(n)