[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.

/**
 * 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 *> s;
    BSTIterator(TreeNode *root) {
        pushLeft(root);
    }
    
    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !s.empty();
    }
    
    /** @return the next smallest number */
    int next() {
        TreeNode *top = s.top();
        s.pop();
        pushLeft(top -> right);
        return top -> val;
    }
    void pushLeft(TreeNode *root){
        if(root){
            s.push(root);
            TreeNode *cur = root;
            while(cur -> left){
                s.push(cur -> left);
                cur = cur -> left;
            }
        }
    }
};




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