Maximum depth of binary tree [https://leetcode.com/problems/maximum-depth-of-binary-tree]

maximum_depth_of_binary_tree.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
 var maxDepth = function(root) {
     if (!root) {
         return 0;
     }
     
    return Math.max(
        getDepthValue(root.left, 1), 
        getDepthValue(root.right, 1)
    );
};

var getDepthValue = function(node, level) {
    if (!node) {
        return level;
    }

    level++;

    return Math.max(
        getDepthValue(node.left, level), 
        getDepthValue(node.right, level)
    );
}