Binary Search Tree Iterator

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    TreeNode* curt ;
    stack<TreeNode*> sdk;


    BSTIterator(TreeNode *root) {
        curt = root;
        //init 
        while(curt != NULL){
            sdk.push(curt);
            curt = curt->left;
        }
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return  !sdk.empty();
    }

    /** @return the next smallest number */
    int next() {

        //pop
        TreeNode * node = sdk.top();
        sdk.pop();
        curt = node->right;

        //push all left child to stack
        //(this code snippet can not move to other position , you gotta push stack immediately after pop stack
        // otherwise the hasnext() will return false )

        while(curt != NULL){
            sdk.push(curt);
            curt = curt->left;
        }
        return node->val;
    }
};

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