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);
}