Add solution and test-cases for problem 880

Author: 0xff-dev

leetcode/801-900/0880.Decoded-String-at-Index/README.md

@@ -1,28 +1,39 @@
 # [880.Decoded String at Index][title]
 
-> [!WARNING|style:flat]
-> This question is temporarily unanswered if you have good ideas. Welcome to [Create Pull Request PR](https://github.com/kylesliu/awesome-golang-algorithm)
-
 ## Description
+You are given an encoded string `s`. To decode the string to a tape, the encoded string is read one character at a time and the following steps are taken:
+
+- If the character read is a letter, that letter is written onto the tape.
+- If the character read is a digit `d`, the entire current tape is repeatedly written `d - 1` more times in total.
+
+Given an integer `k`, return the k<sup>th</sup> letter (**1-indexed**) in the decoded string.
 
 **Example 1:**
 
 ```
-Input: a = "11", b = "1"
-Output: "100"
+Input: s = "leet2code3", k = 10
+Output: "o"
+Explanation: The decoded string is "leetleetcodeleetleetcodeleetleetcode".
+The 10th letter in the string is "o".
 ```
 
-## 题意
-> ...
-
-## 题解
+**Example 2:**
 
-### 思路1
-> ...
-Decoded String at Index
-```go
 ```
+Input: s = "ha22", k = 5
+Output: "h"
+Explanation: The decoded string is "hahahaha".
+The 5th letter is "h".
+```
+
+**Example 3:**
 
+```
+Input: s = "a2345678999999999999999", k = 1
+Output: "a"
+Explanation: The decoded string is "a" repeated 8301530446056247680 times.
+The 1st letter is "a".
+```
 
 ## 结语
 

leetcode/801-900/0880.Decoded-String-at-Index/Solution.go

@@ -1,5 +1,29 @@
 package Solution
 
-func Solution(x bool) bool {
-	return x
+import "fmt"
+
+func Solution(s string, k int) string {
+	size := uint64(0)
+	n := len(s)
+	for i := 0; i < n; i++ {
+		if s[i] >= 'a' && s[i] <= 'z' {
+			size++
+			continue
+		}
+		size = size * uint64(s[i]-'0')
+	}
+	kk := uint64(k)
+	for i := n - 1; i >= 0; i-- {
+		c := s[i]
+		kk %= size
+		if kk == 0 && c >= 'a' && c <= 'z' {
+			return fmt.Sprintf("%c", c)
+		}
+		if c >= 'a' && c <= 'z' {
+			size--
+			continue
+		}
+		size = size / uint64(c-'0')
+	}
+	return ""
 }

leetcode/801-900/0880.Decoded-String-at-Index/Solution_test.go

@@ -10,18 +10,19 @@ func TestSolution(t *testing.T) {
 	//	测试用例
 	cases := []struct {
 		name   string
-		inputs bool
-		expect bool
+		inputs string
+		k      int
+		expect string
 	}{
-		{"TestCase", true, true},
-		{"TestCase", true, true},
-		{"TestCase", false, false},
+		{"TestCase1", "leet2code3", 10, "o"},
+		{"TestCase2", "ha22", 5, "h"},
+		{"TestCase3", "a2345678999999999999999", 1, "a"},
 	}
 
 	//	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.inputs, c.k)
 			if !reflect.DeepEqual(got, c.expect) {
 				t.Fatalf("expected: %v, but got: %v, with inputs: %v",
 					c.expect, got, c.inputs)
@@ -30,10 +31,10 @@ func TestSolution(t *testing.T) {
 	}
 }
 
-//	压力测试
+// 压力测试
 func BenchmarkSolution(b *testing.B) {
 }
 
-//	使用案列
+// 使用案列
 func ExampleSolution() {
 }