230. Kth Smallest Element in a BST

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