Number of Islands LeetCode solution in C++

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class Solution {
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 public:
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  void calculate(vector<vector<char>> &grid, int i, int j) {
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    if (i >= grid.size() || j >= grid[i].size() || i < 0 || j < 0) return;
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    if (grid[i][j] == '0') return;
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    grid[i][j] = '0';
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    calculate(grid, i, j - 1);
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    calculate(grid, i - 1, j);
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    calculate(grid, i, j + 1);
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    calculate(grid, i + 1, j);
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  }
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  int numIslands(vector<vector<char>> &grid) {
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    int ans = 0;
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    for (int i = 0; i < grid.size(); i++) {
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      for (int j = 0; j < grid[0].size(); j++) {
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        if (grid[i][j] == '0')
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          continue;
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        else if (grid[i][j] == '1') {
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          ans++;
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          calculate(grid, i, j);
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        }
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      }
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    }
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    return ans;
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  }
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};