463. Island Perimeter | 岛屿的周长

You are given a map in form of a two-dimensional integer grid where 1 represents land and 0 represents water. Grid cells are connected horizontally/vertically (not diagonally). The grid is completely surrounded by water, and there is exactly one island (i.e.,
one or more connected land cells). The island doesn't have "lakes" (water inside that isn't connected to the water around the island). One cell is a square with side length 1. The grid is rectangular, width and height don't exceed 100. Determine the perimeter
of the island.

Example:

[[0,1,0,0],
[1,1,1,0],
[0,1,0,0],
[1,1,0,0]]

Answer: 16
Explanation: The perimeter is the 16 yellow stripes in the image below:

public int islandPerimeter(int[][] grid) {
int sum = 0;
int n = grid.length;
int m = grid[0].length;
int[] x = new int[] { 1, -1, 0, 0 };
int[] y = new int[] { 0, 0, 1, -1 };
int t = 4;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (grid[i][j] == 1) {
t = 4;
for (int k = 0; k < 4; k++) {
if (i + x[k] >= n || i + x[k] < 0 || j + y[k] >= m || j + y[k] < 0) {

} else if (grid[i + x[k]][j + y[k]] == 1) {
t = t - 1;
} else {

}
}
sum += t;
}
}
}

return sum;
}

其实代码可以写的更简洁些，就是岛屿的相邻岛屿只可能是右边或者下边，然后一个相邻就减去2.