[LeetCode] 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?

中序遍历，Morris，线索二叉树……。

/**
 * 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 cnt = 0;
        TreeNode *p = root;
        stack<TreeNode *> st;
        while (p != NULL || !st.empty()) {
            if (p != NULL) {
                st.push(p);
                p = p->left;
            } else {
                p = st.top();
                st.pop();
                if (++cnt == k) return p->val;
                p = p->right;
            }
        }
    }
};