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Maximum Depth of Binary Tree⚓︎

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Description⚓︎

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:

Example

  • 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⚓︎

Recursive Solution⚓︎

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/**
 * 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));
    }
};

Iterative Solution (Level-order)⚓︎

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/**
 * 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;
        queue<TreeNode*> q;
        q.push(root);
        int depth = 0;

        while (!q.empty()) {
            int len = q.size();
            depth++;
            while (len--) {
                auto curr = q.front();
                q.pop();
                if (curr->left) q.push(curr->left);
                if (curr->right) q.push(curr->right);
            }
        }
        return depth;
    }
};