feat: 5-3

.gitignore

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+*.out

chapter-5/5-3.c

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+/* Exercise 5-3. Write a pointer version of the function strcat that we showed
+ * in Chapter 2:  strcat(s,t) copies the string t to the end of s.  */
+
+#include <ctype.h>
+#include <stdio.h>
+
+// version 1
+void my_strcat1(char *s, char *t) {
+  while (*s)
+    s++; // go to end of s
+
+  while ((*s = *t) != '\0') {
+    s++;
+    t++;
+  }
+}
+
+// version 2
+void my_strcat2(char *s, char *t) {
+  while (*s)
+    s++; // go to end of s
+  while ((*s++ = *t++))
+    ; // then copy
+}
+
+int main() {
+  char s[50] = "Hello, ";
+  char t[] = "world!";
+
+  my_strcat2(s, t);
+
+  printf("%s\n", s);
+
+  return 0;
+}