104. Maximum Depth of Binary Tree

Recursion

/*
 * @lc app=leetcode id=104 lang=cpp
 *
 * [104] Maximum Depth of Binary Tree
 *
 * https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
 *
 * algorithms
 * Easy (76.33%)
 * Likes:    13462
 * Dislikes: 259
 * Total Accepted:    3.9M
 * Total Submissions: 5.1M
 * Testcase Example:  '[3,9,20,null,null,15,7]'
 *
 * Given the root of a binary tree, return its maximum depth.
 *
 * A binary tree's maximum depth is the number of nodes along the longest path
 * from the root node down to the farthest leaf node.
 *
 *
 * Example 1:
 *
 *
 * Input: root = [3,9,20,null,null,15,7]
 * Output: 3
 *
 *
 * Example 2:
 *
 *
 * Input: root = [1,null,2]
 * Output: 2
 *
 *
 *
 * Constraints:
 *
 *
 * The number of nodes in the tree is in the range [0, 10^4].
 * -100 <= Node.val <= 100
 *
 *
 */

// @lc code=start
/**
 * 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:
    int maxDepth(TreeNode* root) {
        if (!root) return 0;
        return 1 + max(maxDepth(root->left), maxDepth(root->right));
    }
};
// @lc code=end

• T: $O(N)$
• S: $O(\log N)$

Iteration

class Solution {
public:
    int maxDepth(TreeNode* root) {
        if (!root) return 0;
        queue<TreeNode*> q{{root}};
        int depth = 0;
        while (!q.empty()) {
            ++depth;
            int qSize = q.size();
            for (int i = 0; i < qSize; ++i) {
                TreeNode* node = q.front(); q.pop();
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
        }
        return depth;
    }
};

• T: $O(N)$
• S: $O(\log N)$