Circle of strings

#include <vector>
#include <string>
#include <unordered_map>

class Solution {
private:
    std::vector<int> parent;
    std::vector<int> rank;

    int find(int x) {
        if (parent[x] != x)
            parent[x] = find(parent[x]);
        return parent[x];
    }

    void unite(int x, int y) {
        int px = find(x), py = find(y);
        if (px == py) return;
        if (rank[px] < rank[py])
            parent[px] = py;
        else if (rank[px] > rank[py])
            parent[py] = px;
        else {
            parent[py] = px;
            rank[px]++;
        }
    }

public:
    int isCircle(std::vector<std::string> &arr) {
        int n = arr.size();
        parent.resize(26);
        rank.resize(26, 0);
        for (int i = 0; i < 26; i++)
            parent[i] = i;

        std::vector<int> inDegree(26, 0), outDegree(26, 0);
        std::unordered_map<int, bool> seen;

        for (const auto &s : arr) {
            int u = s.front() - 'a';
            int v = s.back() - 'a';
            unite(u, v);
            outDegree[u]++;
            inDegree[v]++;
            seen[u] = seen[v] = true;
        }

        int root = -1;
        for (const auto &pair : seen) {
            int vertex = pair.first;
            if (root == -1)
                root = find(vertex);
            else if (find(vertex) != root)
                return 0;
            if (inDegree[vertex] != outDegree[vertex])
                return 0;
        }

        return 1;
    }
};