M102.二叉树的层序遍历

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

思路：利用bfs完成层序遍历，学过二叉树的前中后遍历之后就不算是很困难，但是还是不是很熟练，时间也是大多花在了代码优化，花费半个小时

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
    vector<vector<int>> result; 
        if (root == nullptr) {
            return result;
        }
        
        queue<TreeNode*> q; 
        q.push(root); 
        while (!q.empty()) {
            int levelSize = q.size(); 
            vector<int> currentLevel; 
            
            for (int i = 0; i < levelSize; ++i) {
                TreeNode* node = q.front(); 
                q.pop();
                currentLevel.push_back(node->val); 
                if (node->left != nullptr) {
                    q.push(node->left);
                }
                if (node->right != nullptr) {
                    q.push(node->right);
                }
            }
            
            result.push_back(currentLevel); 
        }
        
        return result;
    }
};