Kth Smallest Element in a BST

基本思想就是：二叉树的中序非递归遍历

/**
 * 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) {
        stack<TreeNode *> st;
        TreeNode * temp=root;
        int res;
        int count=0;
        if(root==NULL)
            return 0;
        while(temp!=NULL||!st.empty())
        {
            if(temp!=NULL)
            {
                st.push(temp);
                if(temp->left!=NULL)
                    temp=temp->left;
                else
                    temp=NULL;
            }
            else if(!st.empty())
            {
                temp=st.top();
                st.pop();
                count++;
                if(count==k)
                {
                    res=temp->val;
                    break;
                }
                temp=temp->right;
            }
        
        }
        return res;
    }
    
};