并查集

class UnionFind {
public:
    UnionFind(vector<vector<char>>& grid) {
        int m = grid.size(), n = grid[0].size();
        parent.assign(m * n, 0);
        rank.assign(m* n, 0);
        count = 0;        // 注意先初始为0
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (grid[i][j] == '1') {
                    parent[i * n + j] = i * n + j;
                    rank[i * n + j] = 1;
                    ++count;        // 陆地块数计数
                } else {
                    parent[i * n + j] = -1;        // 其实可以不加
                }
            }
        }
    }

    int find(int x) {
        if (x != parent[x]) {
            parent[x] = find(parent[x]);        // 路径压缩，递归查询
        }
        return parent[x];
    }

    void unite (int a, int b) {
        int parentA = find(a);
        int parentB = find(b);
        if (parentA != parentB) {
            if (rank[parentA] > rank[parentB]) {        // 按秩合并
                parent[parentB] = parentA;
            } else if (rank[parentA] < rank[parentB]) {
                parent[parentA] = parentB;
            } else {
                parent[parentB] = parentA;
                rank[parentB]++;
            }
            --count;
        }
    }

    int getCount() const {
        return count;
    }
private:
    vector<int> parent;
    vector<int> rank;
    int count;
};

class Solution {
public:
    /**
     * 判断岛屿数量
     * @param grid char字符型vector<vector<>> 
     * @return int整型
     */
    int solve(vector<vector<char> >& grid) {
        // write code here
        int m = grid.size();
        if (m == 0)    return 0;
        int n = grid[0].size();
        UnionFind uf(grid);        // 类初始化
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (grid[i][j] == '1') {
                    grid[i][j] = '0';
                    if (i > 0 && grid[i - 1][j] == '1')    uf.unite(i * n + j, (i - 1) * n + j);
                    if (i + 1 < m && grid[i + 1][j] == '1')    uf.unite(i * n + j, (i + 1) * n + j);
                    if (j > 0 && grid[i][j - 1] == '1')    uf.unite(i * n + j, i * n + j - 1);
                    if (j + 1 < n && grid[i][j + 1] == '1')    uf.unite(i * n + j, i * n + j + 1);
                }
            }
        }
        return uf.getCount();
    }
};