【leetcode】Binary Search Tree Iterator

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.

 

class BSTIterator {
    stack<TreeNode *>s;
public:
    BSTIterator(TreeNode *root) {
        while(!s.empty())
            s.pop();
        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 *tmp=s.top();
        s.pop();
        int ret=tmp->val;
        tmp=tmp->right;
        while(tmp)
        {
            s.push(tmp);
            tmp=tmp->left;
        }

        return ret;
    }
};