LeetCode 173 Binary Search Tree Iterator

题目

mplement 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.

解法

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class BSTIterator {    std::stack<TreeNode*> stack;    int smallest_num;public:    BSTIterator(TreeNode *root) {       while(root != NULL) {            stack.push(root);            root = root->left;        }    }    /** @return whether we have a next smallest number */    bool hasNext() {        if (stack.empty())            return false;        TreeNode* top = stack.top();        stack.pop();        smallest_num = top->val;        TreeNode* temp = top->right;        if(temp != NULL) {            stack.push(temp);            temp = temp->left;            while(temp != NULL) {                stack.push(temp);                temp = temp->left;            }        }        return true;    }    /** @return the next smallest number */    int next() {         return smallest_num;    }};/** * Your BSTIterator will be called like this: * BSTIterator i = BSTIterator(root); * while (i.hasNext()) cout << i.next(); */