leetcode #173 Binary Search Tree Iterator

这题实际上是用BST做排序的算法，应该对BST做中序遍历。

中序遍历：左子树->根节点->右子树

先遍历到的元素用一个栈存起来，方便之后用到的时候把栈中的内容还回来。

程序中的buildStack()就是一个压栈的过程，如果一个节点有左子树，就将左子树压入栈。

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

public class BSTIterator {
    private TreeNode currNode;
    private Stack<TreeNode> stack;
    public BSTIterator(TreeNode root) {
        this.currNode = root;
        this.stack = new Stack<TreeNode>();
        buildStack(root);
    }

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

    /** @return the next smallest number */
    public int next() {
        TreeNode top = stack.pop();
        if(top.right!=null)buildStack(top.right);
        return top.val;

    }
    private void buildStack(TreeNode root) {
        if(root==null)return;
        while(root.left!=null){
            stack.push(root);
            root=root.left;
        }
        stack.push(root);
    }
}

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

Fight On!