can counting contiguous regions in a bitmap be improved over O(r * c)?

Question

You are given an image of a surface photographed by a satellite.The image is a bitmap where water is marked by \'.\' and land is marked by \'*\'. Adjacent group of

Answer 1

Without any apriori knowledge about the nature of islands the algorithm can not be made more efficient than O(n) in time, however memory-wise your algo can be improved. The visited array is simply redundant. Here is a quick attempt (pardon the usage of ASCII artihmetics - not so readable, but quicker to code)

#include 
#define COLS 12


int main()
{
    char map[][COLS] = {
            "*..........",
            "**........*",
            "...........",
            "...*.......",
            "*........*.",
            "..........*"               
            };
    int rows = sizeof(map)/sizeof(map[0]);
    int i, j;  

    int main_count = 0;

    if(map[0][0] == '*') {
        main_count++;
    }
    for(j=0; j