leetcode[230]:Kth Smallest Element in a BST

Kth Smallest Element in a BST

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:

1.Try to utilize the property of a BST.

2.What if you could modify the BST node’s structure?

3.The optimal runtime complexity is O(height of BST).

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     struct TreeNode *left;
*     struct TreeNode *right;
* };
*/

int kthSmallest(struct TreeNode* root, int k) {
struct TreeNode stack[100];
struct TreeNode *tmp;
int i=0;
int l=0;

tmp=root;

while(1)
{

if(tmp->left)
{
stack[i++] = *tmp;
stack[i-1].left=NULL;
tmp=tmp->left;
continue;
}
if(++l==k) return tmp->val;
if(tmp->right)
{
tmp=tmp->right;
continue;
}

if(i==0) break;
tmp = &stack[--i];
}
}

根据中序遍历，遍历到第k个元素即可。