[LeetCode] Kth Smallest Element in a BST

This link suggests a concise C++ recursive solution. The original code may be hard to understand at first and I have rewritten the code below. You may need to run some examples with it to see how it works.

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        return smallest(root, k);
    }
private:
    int smallest(TreeNode* node, int& k) {
        if (!node) return -1;
        int val = smallest(node -> left, k);
        if (!k) return val;
        if (!--k) return node -> val;
        return smallest(node -> right, k);
    }
};

The same author also posts three solutions which is more space-efficient in this link.  All of them reduce the O(n) space of the above code to O(k) space by using some nice data structures.

Personally I really love the soltuion using deque and I have rewritten it below.

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        TreeNode* node = root;
        deque<TreeNode*> nodes;
        while (true) {
            while (node) {
                nodes.push_front(node);
                if (nodes.size() > k)
                    nodes.pop_back();
                node = node -> left;
            }
            node = nodes.front();
            nodes.pop_front();
            if (!--k) return node -> val;
            node = node -> right;
        }
    }
};