LeetCode 230. 二叉搜索树中第K小的元素（中序遍历）

示例 1:

输入: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
输出: 1
示例 2:

输入: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
输出: 3

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
    	int i = 0, ans;
    	bool found = false;
        findkth(root,k,i,ans,found);
        return ans;
    }

    void findkth(TreeNode* &root, int &k, int &i, int &ans, bool &found)
    {
    	if(root == NULL || found)
    		return;
    	findkth(root->left,k,i,ans,found);
    	i++;
    	if(i == k)
		{
			ans = root->val;
			found = true;
		}
    	findkth(root->right,k,i,ans,found);
    }
};

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
		int ans,count = 0;
		stack<TreeNode*> stk;
		while(root || !stk.empty())
		{
			while(root)
			{
				stk.push(root);
				root = root->left;
			}
			if(!stk.empty())
			{
				root = stk.top();
				stk.pop();
				if(++count == k)
					return root->val;
				root = root->right;
			}
		}
		return -1;
	}
};