A-M baby

Task/Find-the-missing-permutation/C/find-the-missing-permutation.c

@@ -0,0 +1,33 @@
+#include <stdio.h>
+
+#define N 4
+const char *perms[] = {
+	"ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",
+	"DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",
+	"DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB",
+};
+
+int main()
+{
+	int i, j, n, cnt[N];
+	char miss[N];
+
+	for (n = i = 1; i < N; i++) n *= i; /* n = (N-1)!, # of occurrence */
+
+	for (i = 0; i < N; i++) {
+		for (j = 0; j < N; j++) cnt[j] = 0;
+
+		/* count how many times each letter occur at position i */
+		for (j = 0; j < sizeof(perms)/sizeof(const char*); j++)
+			cnt[perms[j][i] - 'A']++;
+
+		/* letter not occurring (N-1)! times is the missing one */
+		for (j = 0; j < N && cnt[j] == n; j++);
+
+		miss[i] = j + 'A';
+	}
+	printf("Missing: %.*s\n", N, miss);
+
+	return 0;
+		
+}