[leetcode]Binary Search Tree Iterator

question

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.

answer

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    stack<TreeNode*> *stk;
    BSTIterator(TreeNode *root) {
        stk = new stack<TreeNode*>();
        while (root) {
            stk->push(root);
            root = root->left;
        }
    }
    
    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !stk->empty();
    }
    
    /** @return the next smallest number */
    int next() {
        TreeNode* node = stk->top();
        stk->pop();
        
        int ret = node->val;
        
        if (node->right) {
            node = node->right;
            while (node) {
                stk->push(node);
                node = node->left;
            }
        }
        return ret;
    }
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */