230. Kth Smallest Element in a BST

最新推荐文章于 2020-03-03 16:33:50 发布

转载最新推荐文章于 2020-03-03 16:33:50 发布 · 256 阅读

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#Tree

本文介绍了一个用于查找二叉搜索树中第K小元素的方法。通过使用栈进行迭代，可以有效地找到指定位置的元素。此外，还提供了一种递归方式实现相同功能。这些方法对于频繁查找修改后的二叉搜索树中的特定元素非常有用。

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?

class Solution {
public:
   
        int kthSmallest(TreeNode* root, int k) {
        stack<TreeNode *> st;
        TreeNode *p = root;
        while(p || !st.empty())
        {
            while(p)
            {
                st.push(p);
                p = p->left;
            }
            p = st.top();
            if(--k == 0)
                return p->val;
            st.pop();
            p = p->right;
        }
    }
 
};

const BinaryTreeNode* KthNodeCore(const BinaryTreeNode* pRoot, unsigned int& k)
{
    const BinaryTreeNode* target = nullptr;

    if(pRoot->m_pLeft != nullptr)
        target = KthNodeCore(pRoot->m_pLeft, k);

    if(target == nullptr)
    {
        if(k == 1)
            target = pRoot;

        k--;
    }

    if(target == nullptr && pRoot->m_pRight != nullptr)
        target = KthNodeCore(pRoot->m_pRight, k);

    return target;
}