75 lines
2.2 KiB
C++
75 lines
2.2 KiB
C++
// Source : https://leetcode.com/problems/kth-smallest-element-in-a-bst/
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// Author : Hao Chen
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// Date : 2015-07-03
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/**********************************************************************************
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*
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* Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.
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*
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* Note:
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* You may assume k is always valid, 1 ≤ k ≤ BST's total elements.
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*
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* Follow up:
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* What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently?
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* How would you optimize the kthSmallest routine?
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*
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* Try to utilize the property of a BST.
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* What if you could modify the BST node's structure?
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* The optimal runtime complexity is O(height of BST).
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*
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* Credits:Special thanks to @ts for adding this problem and creating all test cases.
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**********************************************************************************/
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/**
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* Definition for a binary tree node.
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* struct TreeNode {
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* int val;
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* TreeNode *left;
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* TreeNode *right;
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* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
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* };
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*/
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class Solution {
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public:
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// in-order travel - recursive way
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int kthSmallestHelper_recursive(TreeNode* root, int& k) {
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if (root==NULL) return 0; //this behavior is undefined!
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//in-order travel
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int result = kthSmallestHelper_recursive(root->left, k);
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if (k==0) return result;
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k--;
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if (k==0) return root->val;
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return kthSmallestHelper_recursive(root->right, k);
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}
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// in-order travel - non-recursive way
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int kthSmallestHelper_nonRecursive(TreeNode* root, int k){
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stack<TreeNode*> s;
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while(!s.empty() || root){
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while (root) {
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s.push(root);
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root = root->left;
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}
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k--;
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root = s.top()->right;
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if (k==0) return s.top()->val;
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s.pop();
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}
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return -1;
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}
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int kthSmallest(TreeNode* root, int k) {
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//return kthSmallestHelper_nonRecursive(root, k);
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return kthSmallestHelper_recursive(root, k);
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}
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};
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