gold.c

#include <stdio.h>

int data[] = { 3, 4, 6, 5, 11, 2, 19, 1};
int main() {
    int min, when_min, buy, sell;
    int i;
    int profit;
    profit = when_min = buy = sell = 0;
    min = data[0];

    for (i = 0; i < sizeof(data) / sizeof(*data); i++) {
        if (min > data[i]) {
            min = data[i];
            when_min = i;
        }

        if (data[i] - min > profit) {
            profit = data[i] - min;
            buy = when_min;
            sell = i;
        }
    }
    printf("buy on %d for %d, sell on %d for %d, profit %d\n", buy, data[buy], sell, data[sell], profit);
}

gold2.c

#include <stdio.h>

int data[] = { 3, 4, 6, 5, 11, 2, 9, 1};
int main() {
    int *max, *buy, *sell, *ptr;
    int profit;
    profit = 0;
    ptr = max = buy = sell = data + sizeof(data) / sizeof(*data) - 1;

    do {
        if (*max < *ptr)
            max = ptr;

        if (*max - *ptr > profit) {
            profit = *max - *ptr;
            buy = ptr;
            sell = max;
        }
    } while (ptr-- > data);
    printf("buy on %d for %d, sell on %d for %d, profit %d\n", buy - data, *buy, sell - data, *sell, profit);
}