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.

Example:

BSTIterator iterator = new BSTIterator(root);
iterator.next();    // return 3
iterator.next();    // return 7
iterator.hasNext(); // return true
iterator.next();    // return 9
iterator.hasNext(); // return true
iterator.next();    // return 15
iterator.hasNext(); // return true
iterator.next();    // return 20
iterator.hasNext(); // return false

Note:

• next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

• You may assume that next() call will always be valid, that is, there will be at least a next smallest number in the BST when next() is called.

class BSTIterator {
public:
BSTIterator(TreeNode* root) {
    if (root) {
        TreeNode* cur = root;
        find_left(cur);
    }
}

/** @return the next smallest number */
int next() {
    TreeNode* t = st.top(); st.pop();
    int next_smallest = t->val;
    find_left(t->right);
    return next_smallest;
}

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

void find_left(TreeNode* node) {
    while (node) {
        st.push(node);
        node = node->left;
    }
}
stack<TreeNode*> st;
};