【Google面試題】找出二叉尋找樹中出現頻率最高的元素

找出二叉尋找樹中出現頻率最高的元素。樹中結點滿足left->val <= root->val <= right->val。如果多個元素出現次數相等，返回最小的元素。

在一個有序數組中，我們尋找出現頻率最高的元素，很簡單，順序掃描一遍即可統計出。那麼我們對二叉尋找樹也可以用類似方式統計，因為中序遍曆序列就是有序序列，所以我們在中序遍曆的過程中就可以統計出出現頻率最高的元素。

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);}