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.

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这个题剑指offer里也有，简单的递归即可，代码很清晰：

class Solution 
	{
	public:
		int maxDepth(TreeNode *root) 
		{
			if (root == NULL)
			{
				return 0;
			}
			else
			{
				int a = maxDepth(root->left) + 1;
				int b = maxDepth(root->right) + 1;
				if (a > b)
				{
					return a;
				} 
				else
				{
					return b;
				}
			}
		}
	private:
		//static int depth;
	};

Maximum Depth of Binary Tree