LeetCode: 173. Binary Search Tree Iterator

题目描述

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

解题思路

构建中序遍历的线索二叉树(利用哈希表(unordered_map)来存储线索)，线索的顺序就是迭代器遍历的顺序。

AC 代码

/*** Definition for binary tree* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}* };*/
class BSTIterator {
public:BSTIterator(TreeNode *root) {TreeNode* pre = nullptr;makeTreeNextMap(root, pre);tree2Next[pre] = nullptr;curNode = tree2Next[nullptr];}/** @return whether we have a next smallest number */bool hasNext() {return !(curNode == nullptr);}/** @return the next smallest number */int next() {int cur = curNode->val;curNode = tree2Next[curNode];return cur;}
private:void makeTreeNextMap(TreeNode* root, TreeNode*& pre){if(root == nullptr) return ;makeTreeNextMap(root->left, pre);tree2Next[pre] = root;pre=root;makeTreeNextMap(root->right, pre);}private:unordered_map<TreeNode*, TreeNode*> tree2Next; TreeNode* curNode;
};/*** Your BSTIterator will be called like this:* BSTIterator i = BSTIterator(root);* while (i.hasNext()) cout << i.next();*/

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LeetCode: 173. Binary Search Tree Iterator

LeetCode: 173. Binary Search Tree Iterator

题目描述

解题思路

AC 代码

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