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LeetCode-230. Kth Smallest Element in a BST

你的益达233 2022-08-10 阅读 60


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.

Example 1:

Input: root = [3,1,4,null,2], k = 1
3
/ \
1 4
\
2
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
5
/ \
3 6
/ \
2 4
/
1
Output: 3

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?

题解:

注意返回时改变n的值,不然永远返回根节点值。

class Solution {
public:
int n = 1;
void visitTree(TreeNode *t, int k, int &ans) {
if (t->left != NULL) {
visitTree(t->left, k, ans);
}
if (n == k) {
ans = t->val;
n++;
return;
}
n++;
if (t->right != NULL) {
visitTree(t->right, k, ans);
}
}
int kthSmallest(TreeNode* root, int k) {
int ans = 0;
visitTree(root, k, ans);
return ans;
}
};

 

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