How to solve Leetcode 104. Maximum Depth of Binary Tree

Problem statement

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.

Solution

You have the following recursive relationship between the root and its children.

maxDepth(root) = max(maxDepth(root->left), maxDepth(root->right))

Code

#include <iostream>
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) {}
};

int maxDepth(TreeNode* root) {
    if (root == nullptr) {
        return 0;
    }
    return 1 + std::max(maxDepth(root->left), maxDepth(root->right));
}

int main() {
    TreeNode fifteen(15);
    TreeNode seven(7);
    TreeNode twenty(20, &fifteen, &seven);
    TreeNode nine(9);
    TreeNode three(3, &nine, &twenty);
    std::cout << maxDepth(&three) << std::endl;
    TreeNode two(2);
    TreeNode one(1, nullptr, &two);
    std::cout << maxDepth(&one) << std::endl;
}

Output:
3
2

Complexity

Runtime: O(N), where N is the number of nodes.
Extra space: O(N).

References

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