aboutsummaryrefslogtreecommitdiff
path: root/count_and_say.c
diff options
context:
space:
mode:
Diffstat (limited to 'count_and_say.c')
-rw-r--r--count_and_say.c68
1 files changed, 68 insertions, 0 deletions
diff --git a/count_and_say.c b/count_and_say.c
new file mode 100644
index 0000000..8d186fc
--- /dev/null
+++ b/count_and_say.c
@@ -0,0 +1,68 @@
+// 38. Count and Say
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/*
+ * the count-and-say sequence is a sequence of digit strings defined by the
+ * recursive formula:
+ * - countAndSay(1) = "1"
+ * - countAndSay(n) is the way you would "say" the digit string from
+ * 'countAndSay(n - 1)', which is then converted into a different digit string
+ * to determine how you "say" a digit string, split it into minimal number of
+ * substrings such that each substring contains exactly one unique digit. then
+ * for each substring, say the number of digits, then say the digit, finally,
+ * concatenate every said digit. given a positive integer 'n', return the n'th
+ * term of the count and say sequence.
+ */
+
+char *int_to_char(int n) {
+ char *res = malloc(11);
+ res[10] = '\0';
+ int idx = 9;
+ while (n) {
+ res[idx] = '0' + n % 10;
+ n /= 10;
+ idx--;
+ }
+ return &res[idx + 1];
+}
+
+char *countAndSay(int n) {
+ char *curr = malloc(5000), *post = malloc(5000);
+ curr[0] = '1';
+ curr[1] = '\0';
+ if (n == 1)
+ return curr;
+ for (int i = 2; i <= n; i++) {
+ int curr_len = strlen(curr), idx = 0, cnt = 1, pos = 0;
+ char *tmp;
+ for (int j = 1; j < curr_len; j++) {
+ if (curr[j] == curr[j - 1])
+ cnt++;
+ else {
+ tmp = int_to_char(cnt);
+ strncpy(&post[pos], tmp, strlen(tmp));
+ pos += strlen(tmp);
+ post[pos] = curr[j - 1];
+ pos++;
+ cnt = 1;
+ }
+ }
+ tmp = int_to_char(cnt);
+ strncpy(&post[pos], tmp, strlen(tmp));
+ pos += strlen(tmp);
+ post[pos] = curr[curr_len - 1];
+ pos++;
+ post[pos] = '\0';
+ tmp = curr;
+ curr = post;
+ post = tmp;
+ }
+ return curr;
+}
+
+int main() {
+ printf("%s\n", countAndSay(1)); // expect: 1
+ printf("%s\n", countAndSay(4)); // expect: 1211
+}