102. Binary Tree Level Order Traversal

Recursion

/**
 * 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 {
private:
    vector<vector<int>> res;
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        if (!root) return res;
        dfs(root, 0);
        return res;
    }
    void dfs(TreeNode* root, int level) {
        if (res.size() == level) res.emplace_back(vector<int>{});
        res[level].emplace_back(root->val);
        if (root->left) {
            dfs(root->left, level + 1);
        }
        if (root->right) {
            dfs(root->right, level + 1);
        }
    }
};

• T: $O(N)$
• S: $O(N)$

Iteration

/**
 * 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>> res;
        queue<TreeNode*> q{{root}};
        while (!q.empty()) {
            vector<int> nodesWithinLevel;
            int qSize = q.size();
            for (int i = 0; i < qSize; i++) {
                TreeNode* node = q.front(); q.pop();
                if (!node) continue;
                nodesWithinLevel.push_back(node->val);
                q.push(node->left);
                q.push(node->right);
            }
            if (!nodesWithinLevel.empty()) res.push_back(nodesWithinLevel);
        }
        return res;
    }
};

• T: $O(N)$
• S: $O(N)$