463 Island Perimeter

463. Island Perimeter

1. Question

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:

2. Implementation

思路: Everytime the current cell is 1, check its up, down, left and right to see if it is 0, if it is, increase edge by 1

class Solution {
    public int islandPerimeter(int[][] grid) {
        if (grid == null || grid.length == 0) {
            return 0;
        }

        int m = grid.length, n = grid[0].length;
        int edges = 0;

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 1) {
                    // up
                    if (i == 0 || grid[i - 1][j] == 0) edges += 1;
                    // down
                    if (i == m - 1 || grid[i + 1][j] == 0) edges += 1;
                    // left
                    if (j == 0 || grid[i][j - 1] == 0) edges += 1;
                    // right
                    if (j == n - 1 || grid[i][j + 1] == 0) edges += 1;
                }
            }
        }
        return edges;
    }
}

3. Time & Space Complexity

时间复杂度O(mn), 空间复杂度O(mn)