力扣102. 二叉树的层序遍历（非递归法+递归法）

非递归法

/**
 * 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) {
        queue<TreeNode*> que;
        vector<vector<int>> result;
        if(root) que.push(root);
        while(!que.empty()) {
            int size = que.size();
            vector<int> vec;
            while(size--) {
                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;
    }
};

递归法

/**
 * 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:
    void order(TreeNode* node, vector<vector<int>>& result, int depth) {
        if(node == nullptr) return;
        if(result.size() == depth) result.push_back(vector<int>()); // 如果result没有该层，则插入新层
        result[depth].push_back(node->val);
        order(node->left, result, depth + 1);
        order(node->right, result, depth + 1);
    }

    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        order(root, result, 0);
        return result;
    }
};