leetcode:Kth Smallest Element in a B

最新推荐文章于 2020-07-05 11:23:35 发布

martin_liang

最新推荐文章于 2020-07-05 11:23:35 发布

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本文介绍了一种方法来找到给定二叉搜索树中的第K小元素，包括基本实现和后续优化策略。

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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?

Show Hint

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

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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 {
    
private:
    TreeNode* kthSmallestHelper(TreeNode* root, int &k) { // k mest be (&)
        
        if (root == NULL)
            return NULL;
        
        TreeNode *ret = NULL;    
        ret = kthSmallestHelper(root->left, k);
        if (ret != NULL)
            return ret;
        
        k = k-1;
        if (k == 0)
            return root;
            
        ret = kthSmallestHelper(root->right, k);
        if (ret != NULL)
            return ret;
            
        return NULL;
    }
    
public:
    int kthSmallest(TreeNode* root, int k) {
        
        TreeNode* targetNode = kthSmallestHelper(root, k);
        
        return targetNode->val;
    }
};