230. Kth Smallest Element in a BST (中序遍历)

 

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?

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

 

利用中序遍历， 与前面2个题思路一样。

 

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        stack<TreeNode*> stk;
        vector<int> res;
        while(root != nullptr || !stk.empty()) {
            while(root != nullptr) {
                stk.push(root);
                root = root->left;
            }
            root = stk.top();
            stk.pop();
            k--;
            if(k == 0) return root->val;
            root = root->right;
        }
        return 0;

    }
};

 

 

class Solution {
    public int kthSmallest(TreeNode root, int k) {
        if(root==null) return 0;
        TreeNode cur = root;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(cur!=null ||!stack.isEmpty()){
            while(cur!= null){
            stack.push(cur);
            cur = cur.left;
            }
            cur = stack.pop();
            k--;
            if(k==0) return cur.val;
            cur = cur.right;
        }
        return 0;
    }
}