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.
Subscribe to see which companies asked this question.
思路:根据二叉排序树的性质,我用的是中序遍历的非递归形式,访问到一个则k--,知道k==0时则当前访问的即为要找的值。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int kthSmallest(TreeNode root, int k) {
int kth = 0;
LinkedList<TreeNode> list = new LinkedList<>();
while (root != null || !list.isEmpty()) {
while (root != null) {
list.push(root);
root = root.left;
}
TreeNode tmp = list.poll();
k--;
if (k == 0) {
kth = tmp.val;
}
root = tmp.right;
}
return kth;
}
}