20201110: find lost number

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
+}