【一次过】Lintcode 1225. Island Perimeter

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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.

样例

[[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:

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解题思路：

遍历矩阵，若遇到1的情况，则需要判断加四条边的哪几条边，或者不加，这与它的四邻边有关系。具体表现为：如果它左边为0，或者它左边已经是边界的情况，表明存在这条边，周长可以加上1，右上下边类比即可。

public class Solution {
    /**
     * @param grid: a 2D array
     * @return: the perimeter of the island
     */
    public int islandPerimeter(int[][] grid) {
        // Write your code here
        int perimeter = 0;
        
        for(int i=0 ; i<grid.length ; i++){
            for(int j=0 ; j<grid[0].length ; j++){
                if(grid[i][j] == 1){
                    if((j-1)<0 || grid[i][j-1]==0) //左边
                        perimeter++;
                    if((j+1)>=grid[0].length || grid[i][j+1]==0) //右边
                        perimeter++;
                    if((i-1)<0 || grid[i-1][j]==0) //上边
                        perimeter++;
                    if((i+1)>=grid.length || grid[i+1][j]==0) //下边
                        perimeter++;
                }
            }
        }
        
        return perimeter;
    }
}