[LeetCode] Binary Search Tree Iterator

The key to this problem is to store the values in a stack. In the constructor and next, we add all the next smallest nodes into the stack. The following code should be obvious after you run it on some examples.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    BSTIterator(TreeNode *root) {
        pushLeft(root);
    }

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

    /** @return the next smallest number */
    int next() {
        TreeNode* node = nodes.top();
        nodes.pop();
        pushLeft(node -> right);
        return node -> val;
    }
private:
    stack<TreeNode*> nodes;
    void pushLeft(TreeNode* node) {
        while (node) {
            nodes.push(node);
            node = node -> left;
        }
    }
};

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