102. Binary Tree Level Order Traversal 剑指 把二叉树打印成多行

两个题是一样的。

注意一个细节:if(node->left != NULL)  与 if(!node->left) 不一样，即if(!node->left)中感叹号！的优先级高于->left

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        if(!root)
            return result;
        vector<int> res;
        queue<TreeNode*> container;
        container.push(root);
        int now = 1;
        int next = 0;
        while(!container.empty()){
            TreeNode* node = container.front();
            container.pop();
            res.push_back(node->val);
            now--;
            if(node->left != NULL){
                container.push(node->left);
                next++;
            }
            if(node->right != NULL){
                container.push(node->right);
                next++;
            }
            if(now == 0){
                now = next;
                next = 0;
                result.push_back(res);
                res.clear();
            }
        }
        return result;
    }
};