for循环中每次循环内部que.size()会发生变化，导致终止条件也发生变化

/**
 * 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) {
        vector<vector<int>> result;
        if(!root) {
            return result;
        }
        queue<TreeNode*> que;
        que.push(root);
        while(!que.empty()) {
            // 每一层
            vector<int> vec;
            // error, que.size()会变化，应该提前算好int n = que.size()
            for(int i = 0; i < que.size(); i++) {
                TreeNode* node = que.front();
                que.pop();
                vec.push_back(node->val);
                if(node->left) {
                    que.push(node->left);
                }
                if(node->right) {
                    que.push(node->right);
                }
            }
            result.push_back(vec);
        }
        return result;
    }
};