Număr de insule

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

void fill(vector<vector<char>> &a, int i, int j) {
    a[i][j] = '0';
    if (i > 0 && a[i - 1][j] == '1') {
        fill(a, i - 1, j);
    }
    if (i < a.size() - 1 && a[i + 1][j] == '1') {
        fill(a, i + 1, j);
    }
    if (j > 0 && a[i][j - 1] == '1') {
        fill(a, i, j - 1);
    }
    if (j < a[0].size() - 1 && a[i][j+1] == '1') {
        fill(a, i, j + 1);
    }
}
int solve (vector<vector<char>> grid) {
    // TODO : Find number of islands.
    int nr = 0;
    for (int i = 0; i < grid.size(); i++) {
        for (int j = 0; j < grid[i].size(); j++) {
            if (grid[i][j] == '1') {
                nr++;
                fill(grid, i, j);
            }
        }
    }
    return nr;
}


int main() {
    // Don't modify main
    int n, m;
    cin >> n;
    cin >> m;
    vector<vector<char>> grid(n, vector<char>(m));
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            cin >> grid[i][j];
        }
    }
    cout << solve(grid);
    return 0;
}