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.

Analyse: find the next smallest number in the BST and realize the iteration process. The same as inorder traversal.

Runtime: 28ms.

/**
 * 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) {
        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* temp = stk.top();
        stk.pop();
        int nextMin = temp->val;
        temp = temp->right;
        while(temp){
            stk.push(temp);
            temp = temp->left;
        }
        return nextMin;
    }
};

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