triangle.c

// https://stackoverflow.com/questions/53585022/three-colors-triangles/53588144#53588144
// https://www.codewars.com/kata/insane-coloured-triangles/train/c

#define _GNU_SOURCE
#include <stdlib.h>
#include <string.h>

char triangle(const char *s)
{
    char* row = strdup(s);
    size_t strSz = strlen(row);

    while (strSz > 1) {

        char *str = row;

        while (*str != '\0') {
            if      (*str == 'G' && *(str + 1) == 'R') *str = 'B';
            else if (*str == 'R' && *(str + 1) == 'G') *str = 'B';
            else if (*str == 'B' && *(str + 1) == 'G') *str = 'R';
            else if (*str == 'G' && *(str + 1) == 'B') *str = 'R';
            else if (*str == 'B' && *(str + 1) == 'R') *str = 'G';
            else if (*str == 'R' && *(str + 1) == 'B') *str = 'G';

            str++;
        }
        *(str - 1) = '\0';
        strSz--;
    }

    char ch = row[0];
    free(row);
    return ch;
}