Create the-k-th-lexicographical-string-of-all-happy-strings-of-length-n.cpp

Author: kamyu104

C++/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n.cpp

@@ -0,0 +1,21 @@
+// Time:  O(n)
+// Space: O(1)
+
+class Solution {
+public:
+    string getHappyString(int n, int k) {
+        auto base = 1 << (n - 1);
+        if (k > 3 * base) {
+            return "";
+        }
+        string result = string(1, 'a' + (k - 1) / base);
+        while (base > 1) {
+            k -=  ((k - 1) / base) * base;
+            base >>= 1;
+            result += (k - 1) / base == 0
+                      ? (result.back() != 'a' ? 'a' : 'b')
+                      : (result.back() != 'c' ? 'c' : 'b');
+        }
+        return result;
+    }
+};