Solution

• 二叉搜索树，中序有序

/*
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 == nullptr || k <= 0) return nullptr;
        stack<TreeNode *> stack;
        TreeNode* cur = pRoot;
        while (cur || !stack.empty()) {
            while (cur) {
                stack.push(cur);
                cur = cur->left;
            }
            cur = stack.top();
            stack.pop();
            k--;
            if (k == 0) return cur;
            cur = cur->right;
        }
        return nullptr;
    }
};