Add solution and test-cases for problem 1415

leetcode/1401-1500/1415.The-k-th-Lexicographical-String-of-All-Happy-Strings-of-Length-n/README.md

@@ -0,0 +1,44 @@
+# [1415.The k-th Lexicographical String of All Happy Strings of Length n][title]
+
+## Description
+A **happy string** is a string that:
+
+- consists only of letters of the set `['a', 'b', 'c']`.
+- `s[i] != s[i + 1]` for all values of `i` from `1` to `s.length - 1` (string is 1-indexed).
+
+For example, strings **"abc"**, **"ac"**, **"b"** and "abcbabcbcb" are all happy strings and strings **"aa"**, **"baa"** and **"ababbc"** are not happy strings.
+
+Given two integers `n` and `k`, consider a list of all happy strings of length `n` sorted in lexicographical order.
+
+Return the kth string of this list or return an **empty string** if there are less than k happy strings of length n.
+
+**Example 1:**
+
+```
+Input: n = 1, k = 3
+Output: "c"
+Explanation: The list ["a", "b", "c"] contains all happy strings of length 1. The third string is "c".
+```
+
+**Example 2:**
+
+```
+Input: n = 1, k = 4
+Output: ""
+Explanation: There are only 3 happy strings of length 1.
+```
+
+**Example 3:**
+
+```
+Input: n = 3, k = 9
+Output: "cab"
+Explanation: There are 12 different happy string of length 3 ["aba", "abc", "aca", "acb", "bab", "bac", "bca", "bcb", "cab", "cac", "cba", "cbc"]. You will find the 9th string = "cab"
+```
+
+## 结语
+
+如果你同我一样热爱数据结构、算法、LeetCode，可以关注我 GitHub 上的 LeetCode 题解：[awesome-golang-algorithm][me]
+
+[title]: https://leetcode.com/problems/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n
+[me]: https://github.com/kylesliu/awesome-golang-algorithm

leetcode/1401-1500/1415.The-k-th-Lexicographical-String-of-All-Happy-Strings-of-Length-n/Solution.go

@@ -1,5 +1,33 @@
 package Solution
 
-func Solution(x bool) bool {
-	return x
+var canUse = []byte{'a', 'b', 'c'}
+
+func Solution(n int, k int) string {
+	bs := make([]byte, n)
+	var dfs func([]byte, byte, int)
+	cnt := 0
+	dfs = func(cur []byte, pre byte, index int) {
+		if cnt > k {
+			return
+		}
+		if index == n {
+			cnt++
+			if cnt == k {
+				copy(bs, cur)
+			}
+			return
+		}
+		for _, b := range canUse {
+			if b != pre {
+				cur[index] = b
+				dfs(cur, b, index+1)
+			}
+		}
+	}
+	arr := make([]byte, n)
+	dfs(arr, byte(' '), 0)
+	if cnt < k {
+		return ""
+	}
+	return string(bs)
 }

leetcode/1401-1500/1415.The-k-th-Lexicographical-String-of-All-Happy-Strings-of-Length-n/Solution_test.go

@@ -10,30 +10,30 @@ func TestSolution(t *testing.T) {
 	//	测试用例
 	cases := []struct {
 		name   string
-		inputs bool
-		expect bool
+		n, k   int
+		expect string
 	}{
-		{"TestCase", true, true},
-		{"TestCase", true, true},
-		{"TestCase", false, false},
+		{"TestCase1", 1, 3, "c"},
+		{"TestCase2", 1, 4, ""},
+		{"TestCase3", 3, 9, "cab"},
 	}
 
 	//	开始测试
 	for i, c := range cases {
 		t.Run(c.name+" "+strconv.Itoa(i), func(t *testing.T) {
-			got := Solution(c.inputs)
+			got := Solution(c.n, c.k)
 			if !reflect.DeepEqual(got, c.expect) {
-				t.Fatalf("expected: %v, but got: %v, with inputs: %v",
-					c.expect, got, c.inputs)
+				t.Fatalf("expected: %v, but got: %v, with inputs: %v %v",
+					c.expect, got, c.n, c.k)
 			}
 		})
 	}
 }
 
-//	压力测试
+// 压力测试
 func BenchmarkSolution(b *testing.B) {
 }
 
-//	使用案列
+// 使用案列
 func ExampleSolution() {
 }