100 people standing in a circle in the order 1 to 100. No.1 has a sword. He kills the next person (i.e. no. 2 ) and gives sword to next to next (i.e no.3). All persons do the same until only 1 survives. Which number survives at the end?

person_survived.c

#include <stdio.h>

int main()
{
    int n;
    printf("Enter a number (must be <= 100): ");
    scanf("%d", &n);
    
    if (n <= 1) {
        printf("There must be at least two people, my friend!\n");
        return 1;
    }

    /* 
     * static 100 because 
     * "variable-sized object may not be initialized".
     * change array size as per your input range...
     */

    char people[100] = {0};
    char count = n;
    int p = 0;
    int answer;
    while (count != 1) {

        /* do while to validate value of p before compare in condition */

        do {
            if (p >= n)
                p %= n;
        } while (people[p++] != 0);

        /* 
         * because range of people is between 1 to N,
         * not from 0 to N - 1, we assign 'p' to 'answer'.
         * no need to add 1 to 'p'.
         * it is already incremented.
         */

        answer = p;
        do {
            if (p >= n)
                p %= n;
        } while (people[p++] != 0);

        /* decrement count if new index is encounter */
        /* p - 1 because we move one step ahead in last loop 'p++' ;) */

        if (people[p - 1] == 0) {
            count--;
            people[p - 1] = 1;
        }
    }
    printf("ANSWER IS %d\n", answer);
    return 0;
}