the solution to the challenge faced by my friend Umar Faroq

challenge.cpp

#include <stdio.h>
#include <stdlib.h>

int main(){
	
	// prompt:
	printf("please enter a string:");

	// get input
	size_t linelen = 500;	
	char* line = (char*) malloc(linelen);
	if (getline(&line, &linelen, stdin) == -1) {
		printf("Error: no input\n");
		return -1;
	}

	// these are used to track which is the longest
	char* currLongestStart = line;	// points to the start of the substring
	size_t currLongestLen = 1;		// how long the substring is

	// find the longest
	while (*line) {
		char* temp = line++;
			
		// while the next letter is lower in the alphabet
		while (*line && *(line-1) <= *line) {
			line++;
		}
		// if the new one is longer
		if ((line - temp) > currLongestLen) {
			currLongestStart = temp;
			currLongestLen = (line - temp);

		// if they have the same length pick the one with the first letter
		} else if ((line - temp) == currLongestLen && *temp < *currLongestStart) {
			currLongestStart = temp;
			currLongestLen = (line- temp);
		}
	}

	// print the output
	printf("%.*s\n", currLongestLen, currLongestStart);

}