0200.cpp

class UnionFind {
private:
  vector<int> id;
  vector<int> sz;
  int count;

public:
  UnionFind(int N) {
    count = N;
    id.resize(N, 0);
    for (int i = 0; i < N; i++) {
      id[i] = i;
    }
    sz.resize(N, 1);
  }

  ~UnionFind() {}

  int getCount() { return count; }

  bool connected(int p, int q) { return find(p) == find(q); }

  int find(int p) {
    while (p != id[p]) {
      p = id[p];
    }
    return p;
  }

  void doUnion(int p, int q) {
    int p_root = find(p);
    int q_root = find(q);
    if (p_root == q_root) {
      return;
    }

    if (sz[p_root] < sz[q_root]) {
      id[p_root] = q_root;
      sz[q_root] += sz[p_root];
    } else {
      id[q_root] = p_root;
      sz[p_root] += sz[q_root];
    }
    count--;
  }
};

class Solution {
public:
  int numIslands(vector<vector<char>> &grid) {
    // return func1(grid);
    return func2(grid);
  }
  int func1(vector<vector<char>> &grid) {
    int M = grid.size();
    if (M <= 0) {
      return 0;
    }
    int N = grid[0].size();
    UnionFind uf(M * N);

    int count0 = 0;
    for (int i = 0; i < M; i++) {
      for (int j = 0; j < N; j++) {
        if (grid[i][j] == '0') {
          count0++;
          continue;
        }
        if (i - 1 >= 0 && grid[i - 1][j] == '1') {
          uf.doUnion(i * N + j, (i - 1) * N + j);
        }
        if (i + 1 < M && grid[i + 1][j] == '1') {
          uf.doUnion(i * N + j, (i + 1) * N + j);
        }
        if (j - 1 >= 0 && grid[i][j - 1] == '1') {
          uf.doUnion(i * N + j, i * N + j - 1);
        }
        if (j + 1 < N && grid[i][j + 1] == '1') {
          uf.doUnion(i * N + j, i * N + j + 1);
        }
      }
    }
    return uf.getCount() - count0;
  }
  int func2(vector<vector<char>> &grid) {
    int ret = 0;
    for (int i = 0; i < grid.size(); i++) {
      for (int j = 0; j < grid[i].size(); j++) {
        ret += helper2(grid, i, j);
      }
    }
    return ret;
  }
  int helper2(vector<vector<char>> &grid, int i, int j) {
    if (i < 0 || i >= grid.size() || j < 0 || j >= grid[i].size() ||
        grid[i][j] == '0') {

      return 0;
    }

    grid[i][j] = '0';
    helper2(grid, i - 1, j);
    helper2(grid, i + 1, j);
    helper2(grid, i, j - 1);
    helper2(grid, i, j + 1);

    return 1;
  }
};