二叉树层序遍历模板

迭代写法

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vector<vector<int>> levelOrder(TreeNode* root) {
    vector<vector<int>> result;
    queue<TreeNode*> que;
    if (root != NULL) que.push(root);
    while (!que.empty()) {
        int size = que.size();
        vector<int> vec;
        // 这里一定要使用固定大小size，不要使用que.size()，因为que.size()是不断变化的
        for (int i = 0; i < 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;
}

递归写法

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void order(TreeNode* cur, vector<vector<int>>& result, int depth) {
	if (cur == NULL) return;
	
	if (result.size() == depth) result.push_back(vector<int>());
	
	result[depth].push_back(cur->val);
	order(cur->left, result, depth + 1);
	order(cur->right, result, depth + 1);
}
vector<vector<int>> levelOrder(TreeNode* root) {
	vector<vector<int>> result;
	int depth = 0;
	order(root, result, depth);
	return res;
}

题

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

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