leetcode_Kth Smallest Element in a BST

描述：

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:
Try to utilize the property of a BST.Show More Hint

思路：

1.一般涉及到树的问题用树的遍历方式来解决的可能性比较大，或者就是递归了

2.对于本题，一种很朴素的解决方式就是不是二叉查找树么，中序遍历时不就是一个有序的序列么，直接上中序遍历，遍历到第K个数即是本题要求的Kth Smallest Element in a BST 

代码：

public int kthSmallest(TreeNode root, int k) { int count=0;     Stack<TreeNode>st=new Stack<TreeNode>();     st.push(root);     TreeNode top=null;     while(!st.empty())     {     top=st.peek();     while(top.left!=null)     {     st.push(top.left);     top=top.left;     }     while(top.right==null)     {     count++;     if(count==k)        return top.val;     st.pop();     if(!st.empty())     top=st.peek();     else     break;     }     if(!st.empty())     {     count++;     if(count==k)        return top.val;     st.pop();     st.push(top.right);     }          }     return -1;    }