c: ex1: add solution

c/ex1.c

@@ -0,0 +1,26 @@
+#include <stdbool.h>
+#include <stddef.h>
+
+struct node_t {
+    unsigned v;
+    struct node_t* next;
+};
+
+struct node_t* even_nodes(struct node_t** list) {
+    struct node_t* res = NULL;
+
+    while (true) {
+        struct node_t* next = (*list)->next;
+
+        if ((*list)->v % 2 == 0) {
+            // Pop element from the list, add it to head of the other
+            (*list)->next = res;
+            res = *list;
+        }
+
+        *list = next;
+    }
+
+    // NOTE: this is in reverse order from the input list
+    return res;
+}