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, whereh is the height of the tree.

[思路]

非递归遍历BST变种

[注意]

None

[CODE]

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    TreeNode node = null;
    Stack<TreeNode> stack = new Stack<TreeNode>();
    
    public BSTIterator(TreeNode root) {
        node = root;
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {        
        return !( (node==null) && stack.empty() );
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode res = null;

        if(node==null) {
            res = stack.pop();
            node = res.right;
        } else {
            while(node.left!=null) {
                stack.push(node);
                node = node.left;
            }
            res = node;
            node = node.right;
        }
        return res.val;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);e
 * while (i.hasNext()) v[f()] = i.next();
 */

leetcode 173: Binary Search Tree Iterator