Add 1930-unique-length-3-palindromic-subsequences.c

Author: seinlin

c/1930-unique-length-3-palindromic-subsequences.c

@@ -0,0 +1,36 @@
+int countPalindromicSubsequence(char * s){
+    int n = strlen(s);
+    int i, j;
+    int c, answer;
+    int first[26] = {[0 ... 25] = -1};
+    int last[26] = {[0 ... 25] = -1};
+
+    // Use two arrays to record the first and last
+    // location of each character.
+    for (i = 0; i < n; i++) {
+        c = s[i] - 'a';
+        if (first[c] == -1) {
+            first[c] = i;
+        } else {
+            last[c] = i;
+        }
+    }
+
+    // When a character appears twice, count all the
+    // unique character between them.
+    answer = 0;
+    for (i = 0; i < 26; i++) {
+        if (last[i] != -1) {
+            int m[26] = {[0 ... 25] = -1};
+            for (j = first[i] + 1;  j < last[i]; j++) {
+                int c = s[j] - 'a';
+                if (m[c] == -1) {
+                    m[c] = j;
+                    answer++;
+                }
+            }
+        }
+    }
+
+    return answer;
+}