230. Kth Smallest Element in a BST - medium
문제
Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.
제한사항
- You may assume k is always valid, 1 ≤ k ≤ BST’s total elements.
입출력 예
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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
풀이
- BFS
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/**
* 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) {
set<int> s;
queue<TreeNode*> q;
q.push(root);
// BFS로 전체 노드의 val 값을 저장
while(!q.empty()){
auto node = q.front();
q.pop();
s.insert(node->val);
if(node->left)
q.push(node->left);
if(node->right)
q.push(node->right);
}
// k번째 작은수 검색
auto answer = s.begin();
for(auto i = 0 ; i < k - 1 ; ++i){
++answer;
}
return *answer;
}
};