173. Binary Search Tree Iterator -- 迭代器

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 {
    vector<int> v;
    int pos;
public:
    BSTIterator(TreeNode *root) {
        pos = 0;
        stack<TreeNode*> s;
        TreeNode *p = root, *pre = NULL;
        while(p || !s.empty())
        {
            while(p)
            {
                s.push(p);
                p = p->left;
            }
            if(!s.empty())
            {
                p = s.top();
                s.pop();
                v.push_back(p->val);
                p = p->right;
            }
        }
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return pos < v.size();
    }

    /** @return the next smallest number */
    int next() {
        return v[pos++];
    }
};

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