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.
Solution：递归post-order dfs来upwards累积
Time Complexity: O(N) Space Complexity: O(N) 递归缓存
Solution Code：
class Solution {
public int maxDepth(TreeNode root) {
if(root == null) return 0;
return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
}
}
