剑指 Offer 54：二叉搜索树的第k大节点

输入: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
输出: 4

输入: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
输出: 4

class Solution {
public:
    int kthLargest(TreeNode* root, int k) {
        stack<TreeNode*> st;
        TreeNode* cur=root;
        while(!st.empty()||cur){
            while(cur){
                st.push(cur);
                cur=cur->right;
            }
            cur=st.top();
            st.pop();
            if(--k==0) return cur->val;
            cur=cur->left;
        }
        return -1;
    }
};