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 its depth = 3.

1 class Solution {
2 public:
3     int maxDepth(TreeNode* root) {
4         if(!root)return 0;
5         return max(1+maxDepth(root->left),1+maxDepth(root->right));
6     }
7 };

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