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题目：
Given a binary search tree, write a function kthSmallest to find the kth 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?
Hint:
Credits:
Special thanks to @ts for adding this problem and creating all test cases.
代码：
/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: int kthSmallest(TreeNode* root, int k) { int count = 0; stack<TreeNode*> sta; TreeNode* curr = root; while ( !sta.empty() || curr ) { if ( curr ) { sta.push(curr); curr = curr->left; } else { curr = sta.top(); sta.pop(); count++; if ( count==k ) return curr->val; curr = curr->right; } } return curr->val; } };
tips：
直观的做法是中序遍历BST，然后获得第k个元素。
【Kth Smallest Element in a BST 】cpp
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原文地址：http://www.cnblogs.com/xbf9xbf/p/4645159.html
