add new solutions

solutions/14/q1415/solution.go

@@ -0,0 +1,46 @@
+// Package q1415 implements a solution for https://leetcode.com/problems/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n/
+package q1415
+
+import (
+	"math"
+	"strings"
+)
+
+func getHappyString(n, k int) string {
+	// The base is ababababab...
+	// Behaves like binary, except the first character, which has three choices: a, b, c.
+	// Max of k is 2^n + 2^(n-1)
+
+	if k > int(math.Pow(2, float64(n))+math.Pow(2, float64(n-1))) {
+		return ""
+	}
+	k -= 1
+
+	sb := strings.Builder{}
+	var last byte
+	switch k >> (n - 1) {
+	case 0:
+		last = 'a'
+	case 1:
+		last = 'b'
+	case 2:
+		last = 'c'
+	}
+	sb.WriteByte(last)
+
+	lut := [][2]byte{
+		{'b', 'c'},
+		{'a', 'c'},
+		{'a', 'b'},
+	}
+
+	for i := n - 1; i > 0; i-- {
+		lookup := k >> (i - 1) & 1
+		c := lut[last-'a'][lookup]
+		sb.WriteByte(c)
+		last = c
+	}
+	return sb.String()
+}
+
+var _ = getHappyString