173. 二叉搜索树迭代器

通过一个优先队列进行存储数据 然后每一次读数据都是输出第一个结点

class BSTIterator {
	Queue<Integer>ls = new PriorityQueue<Integer>();

	public BSTIterator(TreeNode root) {
        dfs(root);
	}
	public void dfs(TreeNode root) {
		if(root==null) {return ;}
		ls.add(root.val);
		if(root.left!=null)dfs(root.left);
		if(root.right!=null)dfs(root.right);
	}
	/** @return the next smallest number */
	public int next() {
		return ls.poll();
	}

	/** @return whether we have a next smallest number */
	public boolean hasNext() {
		if(ls.size() == 0)
			return false;
		else return true;
	}
}

中序遍历+栈模拟

class BSTIterator {
	Stack<TreeNode> stack = new Stack<TreeNode>();
    public BSTIterator(TreeNode root) {
    	stack.clear();
    	while(root!=null) {
    		stack.add(root);
    		root = root.left;
    	}
    }
    
    /** @return the next smallest number */
    public int next() {
        TreeNode p = stack.pop();
        int res = p .val;
        p = p.right;
        while(p!=null) {
        	stack.add(p);
        	p = p.left;
        }
        return res;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }
}