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题目：

题解：

func numIslands(grid [][]byte) int {
    res := 0
    for i := 0; i < len(grid); i++ {
        for j := 0; j < len(grid[i]); j++ {
            if grid[i][j] == '1' {
                res++
                dfs(grid, i, j)
            }
        }
    }
    return res
}

func dfs(grid [][]byte, r, c int) {
    h, w := len(grid), len(grid[0])
    if r < 0 || r >= h || c < 0 || c >= w {
        return
    }
    if grid[r][c] == '0' {
        return
    }
    grid[r][c] = '0'
    dfs(grid, r-1, c)
    dfs(grid, r+1, c)
    dfs(grid, r, c-1)
    dfs(grid, r, c+1)
}