leetcode-230. 二叉搜索树中第K小的元素-中序遍历

1、采用二叉树的中序遍历，先把所有的左子树添加进去，再开始计数到k

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        stack stack;
        while (root != nullptr || stack.size() > 0) {
            while (root != nullptr) {
                stack.push(root);
                root = root->left;
            }
            root = stack.top();
            stack.pop();
            --k;
            if (k == 0) {
                break;
            }
            root = root->right;
        }
        return root->val;
    }
};