Lowest Common Ancestor of a Binary Search Tree有问题

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class NewTypeNode{
public:
    TreeNode * present;
    TreeNode * father;
    NewTypeNode(TreeNode * p,TreeNode * f):present(p),father(f) {}
}
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        
        if(root==NULL)
            return ;
        stack<TreeNode *> st;
        st.push(root);
        NewTypeNode head=NewTypeNode(root,NULL);
        while(!st.empty())
        {
            TreeNode * temp=st.top();
            st.pop();
            
            if(temp->left!=NULL)
                st.push(temp->left);
            if(temp->right!=NULL)
                st.push(temp->right);
                
        }
        
    }
};