找出二叉查找树中出现频率最高的元素。树中结点满足left->val <= root->val <= right->val。如果多个元素出现次数相等，返回最小的元素。

在一个有序数组中，我们查找出现频率最高的元素，很简单，顺序扫描一遍即可统计出。那么我们对二叉查找树也可以用类似方式统计，因为中序遍历序列就是有序序列，所以我们在中序遍历的过程中就可以统计出出现频率最高的元素。

class TreeNode
{
public:
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int val, TreeNode* left = NULL, TreeNode *right = NULL)
	{
		this->val = val;
		this->left = left;
		this->right = right;
	}
};

int GetMostFrequently(TreeNode * root)
{
	void _GetMostFrequently(TreeNode *root, int & current, int & currentFrequency, 
				int & maxFrequency, int & mostFrequently);
	if(root == NULL)
		throw new invalid_argument("Can't be a NULL tree");
	int mostFrequently = INT_MAX;
	int current = INT_MAX;
	int currentFrequency = 0;
	int maxFrequency = 0;
	_GetMostFrequently(root, current, currentFrequency, maxFrequency, mostFrequently);
	return mostFrequently;
}

void _GetMostFrequently(TreeNode *root, int & current, int & currentFrequency, 
			int & maxFrequency, int & mostFrequently)
{
	if(root == NULL)
		return;
	_GetMostFrequently(root->left, current, currentFrequency, 
				maxFrequency, mostFrequently);
	if(root->val == current)
		++currentFrequency;
	else
	{
		current = root->val;
		currentFrequency = 1;
	}
	if(currentFrequency > maxFrequency)
	{
		maxFrequency = currentFrequency;
		mostFrequently = current;
	}
	_GetMostFrequently(root->right, current, currentFrequency, 
				maxFrequency, mostFrequently);
}