104. Maximum Depth of Binary Tree

Question

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, 104].
-100 <= Node.val <= 100

---

Approach

1. Starting from the root, recursively travel to the leaf node and return 0 because that means its on the 0th level.
2. On every level, check the depth of left and right node respectively.
3. Returns the larger depth + 1 (since the level starts from 0, but we need the number of levels which is > 0)

Solution

Go

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func maxDepth(root *TreeNode) int {
    if root == nil{
        return 0
    }else{
        lDepth := maxDepth(root.Left)
        rDepth := maxDepth(root.Right)
        
        if lDepth > rDepth{
            return lDepth + 1
        }
        return rDepth + 1
    }
}

C++

/**
 * 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 == NULL) return 0;
        else {
            int lDepth = maxDepth(root->left);
            int rDepth = maxDepth(root->right);

            if (lDepth > rDepth) return lDepth+1;
            return rDepth + 1;
        }
    }
};