LeetCode 230 Kth Smallest Element in a BST 二叉搜索树中的第K个元素

1、非递归解法

 /**
  * Definition for a binary tree node.
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     int kthSmallest(TreeNode* root, int k) {
         if(root==nullptr)
             return -;
         stack<TreeNode*> stk;
         while(root||!stk.empty())
         {
             if(root)
             {
                 stk.push(root);
                 root=root->left;
             }
             else
             {
                 root=stk.top();
                 stk.pop();
                 if(k==)
                     return root->val;
                 --k;
                 root=root->right;
             }
         }
         return -;
     }
 };

2、递归解法

 /**
  * Definition for a binary tree node.
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     int kthSmallest(TreeNode* root, int k) {
         return helper(root, k);
     }
     int helper(TreeNode* root, int &k) {
         if (!root)
             return -;
         int val = helper(root->left, k);
         if (k == )
             return val;
         if (--k == )
             return root->val;
         return helper(root->right, k);
     }
 };