[LeetCode-230]Kth Smallest Element in a BST

原创于 2015-12-27 14:10:03 发布 · 381 阅读

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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，包括基本实现和针对频繁修改的优化策略。

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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?

Solution：

/**
 * 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 tree_size(TreeNode* root){
        if (root){
            return tree_size(root->left) + tree_size(root->right) + 1;
        } else {
            return 0;
        }
    }
    int kthSmallest(TreeNode* root, int k) {
        if (tree_size(root->left) + 1 == k) {
            return root->val;
        } else if (tree_size(root->left) + 1 > k) {
            return kthSmallest(root->left, k);
        } else {
            return kthSmallest(root->right, k-(tree_size(root->left)+1));
        }
    }
};