Merge pull request #990 from 0xff-dev/400

Add solution and test-cases for problem 400

leetcode/301-400/0400.Nth-Digit/README.md

@@ -0,0 +1,26 @@
+# [400. Nth Digit][title]
+
+## Description
+Given an integer `n`, return the `nth` digit of the infinite integer sequence `[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ...]`.
+
+**Example 1:**
+
+```
+Input: n = 3
+Output: 3
+```
+
+**Example 2:**
+
+```
+Input: n = 11
+Output: 0
+Explanation: The 11th digit of the sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ... is a 0, which is part of the number 10.
+```
+
+## 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/nth-digit
+[me]: https://github.com/kylesliu/awesome-golang-algorithm

leetcode/301-400/0400.Nth-Digit/Solution.go

@@ -1,5 +1,28 @@
 package Solution
 
-func Solution(x bool) bool {
-	return x
+func Solution(n int) int {
+	if n <= 9 {
+		return n
+	}
+	fixed := 9
+	base, pow := 1, 1
+	cur := base * pow * fixed
+	for cur < n {
+		n -= cur
+		base, pow = base+1, pow*10
+		cur = base * pow * fixed
+	}
+	need := n / base
+	target := pow + (need - 1)
+	mod := n % base
+	if mod == 0 {
+		return target % 10
+	}
+
+	shift := base - mod
+	target++
+	for ; shift > 0; shift-- {
+		target /= 10
+	}
+	return target % 10
 }

leetcode/301-400/0400.Nth-Digit/Solution_test.go

@@ -10,12 +10,11 @@ func TestSolution(t *testing.T) {
 	//	测试用例
 	cases := []struct {
 		name   string
-		inputs bool
-		expect bool
+		inputs int
+		expect int
 	}{
-		{"TestCase", true, true},
-		{"TestCase", true, true},
-		{"TestCase", false, false},
+		{"TestCase1", 3, 3},
+		{"TestCase2", 11, 0},
 	}
 
 	//	开始测试
@@ -30,10 +29,10 @@ 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() {
 }