二叉搜索树的第k个结点

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(!pRoot || k <= 0)
            return NULL;
        
        stack<TreeNode *> stk;
        TreeNode *res = NULL;
        
        TreeNode *p = pRoot;
        int index = 0;
        
        while(p || !stk.empty()){
            while(p){
                stk.push(p);
                p = p->left;
            }
            
            if(!stk.empty()){
                p = stk.top();
                stk.pop();
                
                index++;
                
                if(index == k){
                    res = p;
                    break;
                }
                
                p = p->right;
            }
        }
        
        return res;
    }
};