LeetCode 230: Kth Smallest Element in a BST，leetcodekth

LeetCode 230: Kth Smallest Element in a BST，leetcodekth

Given a binary search tree, write a function kthSmallest to find thekth smallest element in it.

Note:
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

在二叉搜尋樹種，找到第K個元素。

演算法如下：

1、計算左子樹元素個數left。

2、 left+1 = K，則根節點即為第K個元素

      left >=k, 則第K個元素在左子樹中，

     left +1 <k, 則轉換為在右子樹中，尋找第K-left-1元素。

代碼如下：

class Solution {public:int calcTreeSize(TreeNode* root){if (root == NULL)return 0;return 1+calcTreeSize(root->left) + calcTreeSize(root->right);}int kthSmallest(TreeNode* root, int k) {if (root == NULL)return 0;int leftSize = calcTreeSize(root->left);if (k == leftSize+1){return root->val;}else if (leftSize >= k){return kthSmallest(root->left,k);}else{return kthSmallest(root->right, k-leftSize-1);}}};

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