leetcode笔记-Kth Smallest Element in a BST

思路：定义一个中序遍历的函数，调用中序遍历将各个节点都存在VECTOR中，其中的数是从小到大的排列的，再取第K个最小的值

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };//中序遍历类似
 */
class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        int c;
        if(root==NULL) c=0;
        else inorder(root,k,0,c);
        return c;
    }
  
    /*int inorder(TreeNode* root,int k)
    {   vector<int> a;
        stack<TreeNode*> s;
        int l=0;
        if(root==NULL) return 0;
        while(!s.empty()||root!=NULL)
        {
            while(!root)
            {
                s.push(root);
                root=root->left;
            }
            root=s.top();
            s.pop();
            a.push_back(root->val);
            l++;
            if(l==k)  break;
            root=root->right;
        }
        return root->val;
    }
    */
    void inorder(TreeNode* root,int k,int a,int res)
    {
        if(root==NULL||a>k) return;
        inorder(root->left,k,a,res);
        a++;
        if(a==k) res==root->val;
        inorder(root->right,k,a,res);
    }
};