Binary Search Tree Iterator —— Leetcode

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.

设计一个迭代器，关键在于O(h)的空间复杂度，用一个栈实现。

/** * 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) {        while(root) {            s.push(root);            root = root->left;        }    }    /** @return whether we have a next smallest number */    bool hasNext() {        return !s.empty();    }    /** @return the next smallest number */    int next() {        TreeNode* node = s.top();        s.pop();        if(node->right) {            TreeNode* rightNode = node->right;            s.push(rightNode);            while(rightNode->left) {                s.push(rightNode->left);                rightNode = rightNode->left;            }        }        return node->val;    }        private:        stack<TreeNode*> s;};/** * Your BSTIterator will be called like this: * BSTIterator i = BSTIterator(root); * while (i.hasNext()) cout << i.next(); */