Task: Ulam numbers
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

void fatal(const char* message) {
    fprintf(stderr, "%s\n", message);
    exit(1);
}

void* xmalloc(size_t n) {
    void* ptr = malloc(n);
    if (ptr == NULL)
        fatal("Out of memory");
    return ptr;
}

void* xrealloc(void* p, size_t n) {
    void* ptr = realloc(p, n);
    if (ptr == NULL)
        fatal("Out of memory");
    return ptr;
}

int* extend(int* array, int min_length, int* capacity) {
    int new_capacity = *capacity;
    if (new_capacity >= min_length)
        return array;
    while (new_capacity < min_length)
        new_capacity *= 2;
    array = xrealloc(array, new_capacity * sizeof(int));
    memset(array + *capacity, 0, (new_capacity - *capacity) * sizeof(int));
    *capacity = new_capacity;
    return array;
}

int ulam(int n) {
    int* ulams = xmalloc((n < 2 ? 2 : n) * sizeof(int));
    ulams[0] = 1;
    ulams[1] = 2;
    int sieve_length = 2;
    int sieve_capacity = 2;
    int* sieve = xmalloc(sieve_capacity * sizeof(int));
    sieve[0] = sieve[1] = 1;
    for (int u = 2, ulen = 2; ulen < n; ) {
        sieve_length = u + ulams[ulen - 2];
        sieve = extend(sieve, sieve_length, &sieve_capacity);
        for (int i = 0; i < ulen - 1; ++i)
            ++sieve[u + ulams[i] - 1];
        for (int i = u; i < sieve_length; ++i) {
            if (sieve[i] == 1) {
                u = i + 1;
                ulams[ulen++] = u;
                break;
            }
        }
    }
    int result = ulams[n - 1];
    free(ulams);
    free(sieve);
    return result;
}

int main() {
    clock_t start = clock();
    for (int n = 1; n <= 100000; n *= 10)
        printf("Ulam(%d) = %d\n", n, ulam(n));
    clock_t finish = clock();
    printf("Elapsed time: %.3f seconds\n", (finish - start + 0.0)/CLOCKS_PER_SEC);
    return 0;
}

Java: public class UlamNumbers {
    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        for (int n = 1; n <= 100000; n *= 10) {
            System.out.printf("Ulam(%d) = %d\n", n, ulam(n));
        }
        long finish = System.currentTimeMillis();
        System.out.printf("Elapsed time: %.3f seconds\n", (finish - start)/1000.0);
    }

    private static int ulam(int n) {
        int[] ulams = new int[Math.max(n, 2)];
        ulams[0] = 1;
        ulams[1] = 2;
        int sieveLength = 2;
        int[] sieve = new int[sieveLength];
        sieve[0] = sieve[1] = 1;
        for (int u = 2, ulen = 2; ulen < n; ) {
            sieveLength = u + ulams[ulen - 2];
            sieve = extend(sieve, sieveLength);
            for (int i = 0; i < ulen - 1; ++i)
                ++sieve[u + ulams[i] - 1];
            for (int i = u; i < sieveLength; ++i) {
                if (sieve[i] == 1) {
                    u = i + 1;
                    ulams[ulen++] = u;
                    break;
                }
            }
        }
        return ulams[n - 1];
    }

    private static int[] extend(int[] array, int minLength) {
        if (minLength <= array.length)
            return array;
        int newLength = 2 * array.length;
        while (newLength < minLength)
            newLength *= 2;
        int[] newArray = new int[newLength];
        System.arraycopy(array, 0, newArray, 0, array.length);
        return newArray;
    }
}