Leetcode230:二叉搜索树中第K小的元素

	int count = 0;

	//解法1 中序遍历
	public int kthSmallest(TreeNode root, int k) {

		if (root == null) {
			return Integer.MIN_VALUE;
		}
		int left = kthSmallest(root.left, k);
		if (left!=Integer.MIN_VALUE){
			return left;
		}
		count++;
		if (count==k){
			return root.val;
		}
		int right = kthSmallest(root.right, k);
		if (right!=Integer.MIN_VALUE){
			return right;
		}
		return Integer.MIN_VALUE;
	}

	//解法2 中序遍历非递归法
	public int kthSmallest2(TreeNode root, int k) {
		if (root==null){
			throw new IllegalArgumentException();
		}
		Stack<TreeNode> stack = new Stack<>();
		while (!stack.isEmpty() || root!=null) {
			while (root!=null){
				stack.push(root);
				root = root.left;
			}
			root = stack.pop();
			k--;
			if (k==0){
				return root.val;
			}
			root = root.right;
		}
		throw new IllegalArgumentException();
	}