問題描述
Given a binary tree, find its maximum depth.
The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
Note: A leaf is a node with no children.
Example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return depth 3
思路：遞歸咯，判斷每個節(jié)點的左子樹和右子樹的高度

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */

function depth(root) {
    if(root == null) return 0;
    return Math.max(depth(root.left), depth(root.right)) + 1
}

var maxDepth = function(root) {
    return depth(root)
};