From 37ac2b7c2b2a2ad394cb7f4f574da5f73867d6d4 Mon Sep 17 00:00:00 2001
From: VicRen <vic.ren@me.com>
Date: Tue, 10 Nov 2020 14:17:18 +0800
Subject: [PATCH] 20201110: find lost number

---
 20201110_find_lost_number/main.go      | 38 ++++++++++++++++
 20201110_find_lost_number/main_test.go | 63 ++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)
 create mode 100644 20201110_find_lost_number/main.go
 create mode 100644 20201110_find_lost_number/main_test.go

diff --git a/20201110_find_lost_number/main.go b/20201110_find_lost_number/main.go
new file mode 100644
index 0000000..0988795
--- /dev/null
+++ b/20201110_find_lost_number/main.go
@@ -0,0 +1,38 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+)
+
+func main() {
+	r := rand.Intn(100)
+	fmt.Println("r:", r)
+	var src []int
+	for i := 0; i < 100; i++ {
+		if i == r {
+			continue
+		}
+		src = append(src, i)
+	}
+	fmt.Println("lost number:", findLostNumber(src))
+	fmt.Println("lost number2:", findLostNumber2(src))
+}
+
+func findLostNumber(nums []int) int {
+	sum := 0
+	for _, n := range nums {
+		sum += n
+	}
+	n := len(nums)
+	return n*(n+1)/2 - sum
+}
+
+func findLostNumber2(nums []int) int {
+	res := 0
+	for i, n := range nums {
+		res = res ^ i ^ n
+	}
+	res = res ^ len(nums)
+	return res
+}
diff --git a/20201110_find_lost_number/main_test.go b/20201110_find_lost_number/main_test.go
new file mode 100644
index 0000000..9337bd1
--- /dev/null
+++ b/20201110_find_lost_number/main_test.go
@@ -0,0 +1,63 @@
+package main
+
+import (
+	"testing"
+)
+
+func Test_findLostNumber(t *testing.T) {
+	type args struct {
+		nums []int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{
+			"example_1",
+			args{[]int{3, 0, 1}},
+			2,
+		},
+		{
+			"example_2",
+			args{[]int{9, 6, 4, 2, 3, 5, 7, 0, 1}},
+			8,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := findLostNumber(tt.args.nums); got != tt.want {
+				t.Errorf("findLostNumber() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_findLostNumber2(t *testing.T) {
+	type args struct {
+		nums []int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{
+			"example_1",
+			args{[]int{3, 0, 1}},
+			2,
+		},
+		{
+			"example_2",
+			args{[]int{9, 6, 4, 2, 3, 5, 7, 0, 1}},
+			8,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := findLostNumber2(tt.args.nums); got != tt.want {
+				t.Errorf("findLostNumber2() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}