Add merge sort.

merge.c

@@ -0,0 +1,61 @@
+#include <stdio.h>
+#include <math.h>
+#include "utils.c"
+
+// merge sort
+void merge(int *items, int *tmp, int p, int q, int r);
+void mergeSort(int *items, int *tmp, int p, int r);
+
+int main(int argc, char *argv[])
+{
+  int itemCount = 9;
+  int items[9] = {8, 7, 6, 5, 4, 3, 2, 1, 0};
+  int tmp[9] = {};
+
+  printInts(items, itemCount);
+  mergeSort(items, tmp, 0, itemCount - 1);
+  printInts(items, itemCount);
+
+  return 0;
+}
+
+void mergeSort(int *items, int *tmp, int p, int r)
+{
+  if(p < r) {
+    int q = floor((p + r) / 2);
+    mergeSort(items, tmp, p, q);
+    mergeSort(items, tmp, q + 1, r);
+    merge(items, tmp, p, q, r);
+  }
+}
+
+void merge(int *items, int *tmp, int p, int q, int r) {
+  int aCount = p;
+  int bCount = q + 1;
+
+  int i;
+
+  // read numbers to be merged into tmp
+  for(i = p; i <= r; i++) {
+    tmp[i] = items[i];
+  }
+
+  for(i = p; i <= r; i++) {
+    if(aCount > q) {
+      items[i] = tmp[bCount];
+      bCount++;
+    }
+    else if(bCount > r) {
+      items[i] = tmp[aCount];
+      aCount++;
+    }
+    else if(tmp[aCount] <= tmp[bCount]) {
+      items[i] = tmp[aCount];
+      aCount++;
+    }
+    else if(tmp[bCount] < tmp[aCount]) {
+      items[i] = tmp[bCount];
+      bCount++;
+    }
+  }
+}