geeksforgeeks: add lexicographically next string

geeksforgeeks/lexicographically-next-string/README.md

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+# Lexicographically next string
+
+Given a string, find lexicographically next string.
+If string is empty, we return ‘a’.
+If string contains all characters as ‘z’, we append ‘a’ at the end.
+Otherwise we find first character from end which is not z and increment it.
+
+## Copyright Notice
+
+This problem is based on [content](https://www.geeksforgeeks.org/lexicographically-next-string/)
+from [GeeksforGeeks](https://www.geeksforgeeks.org)
+written by GeeksforGeeks
+and subject to [GeeksforGeeks copyright](https://www.geeksforgeeks.org/legal/copyright-information/).
+The original content from GeeksforGeeks and any modifications made here are attributed to GeeksforGeeks contributors,
+and this work is shared under [CC BY-SA 4.0](../LICENSE).

geeksforgeeks/lexicographically-next-string/solution.go

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+package lexicographically_next_string
+
+import "strings"
+
+func Solution(s string) string {
+	i := len(s) - 1
+	for i >= 0 && s[i] == 'z' {
+		i--
+	}
+
+	if i == -1 {
+		return s + "a"
+	}
+
+	j := 0
+	return strings.Map(func(r rune) rune {
+		if j == i {
+			r += 1
+		}
+		j++
+		return r
+	}, s)
+}

geeksforgeeks/lexicographically-next-string/solution_test.go

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+package lexicographically_next_string_test
+
+import (
+	"testing"
+
+	sut "github.com/minizilla/minmax/geeksforgeeks/lexicographically-next-string"
+	"github.com/minizilla/testr"
+)
+
+func TestLexicographicallyNextString(t *testing.T) {
+	tests := map[string]struct {
+		in, out string
+	}{
+		"empty": {"", "a"},
+		"a":     {"a", "b"},
+		"z":     {"z", "za"},
+		"ab":    {"ab", "ac"},
+		"az":    {"az", "bz"},
+		"zz":    {"zz", "zza"},
+	}
+
+	for name, tc := range tests {
+		t.Run(name, func(t *testing.T) {
+			assert := testr.New(t)
+			out := sut.Solution(tc.in)
+			assert.Equal(out, tc.out)
+		})
+	}
+}