[宽度优先搜索] leetcode 102 Binary Tree Level Order Traversal

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if(!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty())
        {
            vector<int> layer;
            int size = q.size();
            while(size--)
            {
                TreeNode* cur = q.front();
                q.pop();
                
                layer.push_back(cur->val);
                if(cur->left) q.push(cur->left);
                if(cur->right) q.push(cur->right);
            }
            res.push_back(layer);
        }
        return res;
    }
};