Add C solution

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diff --git a/challenge-023/paulo-custodio/c/ch-1.c b/challenge-023/paulo-custodio/c/ch-1.c
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+/*
+Challenge 023
+
+Task #1
+Create a script that prints nth order forward difference series. You should
+be a able to pass the list of numbers and order number as command line
+parameters. Let me show you with an example.
+
+Suppose we have list (X) of numbers: 5, 9, 2, 8, 1, 6 and we would like to
+create 1st order forward difference series (Y). So using the formula
+Y(i) = X(i+1) - X(i), we get the following numbers:
+(9-5), (2-9), (8-2), (1-8), (6-1).
+In short, the final series would be: 4, -7, 6, -7, 5.
+If you noticed, it has one less number than the original series.
+Similarly you can carry on 2nd order forward difference series like:
+(-7-4), (6+7), (-7-6), (5+7) => -11, 13, -13, 12.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+void* check_mem(void* p) {
+    if (!p) {
+        fputs("Out of memory", stderr);
+        exit(EXIT_FAILURE);
+    }
+    return p;
+}
+
+int forward_diff(int* nums, int size) {
+    if (size <= 1)
+        return 0;
+
+    int* diff = check_mem(malloc((size-1) * sizeof(int)));
+    for (int i = 0; i < size-1; i++)
+        diff[i] = nums[i+1] - nums[i];
+
+    memcpy(nums, diff, (size-1) * sizeof(int));
+    free(diff);
+    return size-1;
+}
+
+int nth_forward_diff(int n, int* nums, int size) {
+    for (int i = 0; i < n; i++)
+        size = forward_diff(nums, size);
+    return size;
+}
+
+int main(int argc, char* argv[]) {
+    if (argc < 3) {
+        fputs("usage: ch-1 nums\n", stderr);
+        return EXIT_FAILURE;
+    }
+
+    int n = atoi(argv[1]);
+    int size = argc-2;
+    int* nums = check_mem(malloc(size * sizeof(int)));
+    for (int i = 0; i < size; i++)
+        nums[i] = atoi(argv[i+2]);
+
+    size = nth_forward_diff(n, nums, size);
+
+    for (int i = 0; i < size; i++)
+        printf("%s%d", i==0 ? "" : ", ", nums[i]);
+    printf("\n");
+
+    free(nums);
+}