173.binary-search-tree-iterator.0.cpp

/*
 * @lc app=leetcode id=173 lang=cpp
 *
 * [173] Binary Search Tree Iterator
 *
 * https://leetcode.com/problems/binary-search-tree-iterator/description/
 *
 * algorithms
 * Medium (50.30%)
 * Total Accepted:    225.3K
 * Total Submissions: 447.9K
 * Testcase Example:  '["BSTIterator","next","next","hasNext","next","hasNext","next","hasNext","next","hasNext"]\n[[[7,3,15,null,null,9,20]],[null],[null],[null],[null],[null],[null],[null],[null],[null]]'
 *
 * 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.
 * 
 * 
 * 
 * 
 * 
 * 
 * Example:
 * 
 * 
 * 
 * 
 * BSTIterator iterator = new BSTIterator(root);
 * iterator.next();    // return 3
 * iterator.next();    // return 7
 * iterator.hasNext(); // return true
 * iterator.next();    // return 9
 * iterator.hasNext(); // return true
 * iterator.next();    // return 15
 * iterator.hasNext(); // return true
 * iterator.next();    // return 20
 * iterator.hasNext(); // return false
 * 
 * 
 * 
 * 
 * Note:
 * 
 * 
 * next() and hasNext() should run in average O(1) time and uses O(h) memory,
 * where h is the height of the tree.
 * You may assume that next() call will always be valid, that is, there will be
 * at least a next smallest number in the BST when next() is called.
 * 
 * 
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
	stack<TreeNode*> st;
	void fillme(TreeNode* root){
		while(root!=NULL)
			st.push(root), root = root->left;
	}
    BSTIterator(TreeNode* root) {
    	assert(st.empty());
        fillme(root);
    }
    
    /** @return the next smallest number */
    int next() {
    	TreeNode* top = st.top(); st.pop();
    	fillme(top->right);
    	return top->val;   
    }
    
    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !st.empty();
    }
};

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator* obj = new BSTIterator(root);
 * int param_1 = obj->next();
 * bool param_2 = obj->hasNext();
 */