leetcode[173]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 {
private:
    stack<TreeNode *> sta;
public:
void leftOrder(TreeNode *root)
{
    if(root==NULL)return;
    TreeNode *tmp=root;
    while(tmp)
    {
        sta.push(tmp);
        if(tmp->left)
        {
            tmp=tmp->left;
        }
        else break;
    }
    return;
}
    BSTIterator(TreeNode *root) {
      leftOrder(root); 
    }

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

    /** @return the next smallest number */
    int next() {
        TreeNode *top=sta.top();
        sta.pop();
        if(top->right)leftOrder(top->right);
        return top->val;
    }
};

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