37: truncatable primes

37_truncatable_primes/main.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+func main() {
+	sum := 0
+	start := 8
+	count := 0
+	for {
+		if count == 11 {
+			break
+		}
+		if isTruncatablePrimesL2R(start) && isTruncatablePrimesR2L(start) {
+			count++
+			fmt.Println("tp:", start)
+			sum += start
+		}
+		start++
+	}
+	fmt.Println("sum:", sum)
+}
+
+func isTruncatablePrimesL2R(n int) bool {
+	s := strconv.Itoa(n)
+	for len(s) > 0 {
+		x, _ := strconv.Atoi(s)
+		if !isPrime(x) {
+			return false
+		}
+		s = s[1:]
+	}
+	return true
+}
+
+func isTruncatablePrimesR2L(n int) bool {
+	s := strconv.Itoa(n)
+	for len(s) > 0 {
+		l := len(s)
+		x, _ := strconv.Atoi(s)
+		if !isPrime(x) {
+			return false
+		}
+		s = s[:l-1]
+	}
+	return true
+}
+
+func isPrime(n int) bool {
+	if n <= 1 {
+		return false
+	}
+	res := n
+	//牛顿法求平方根
+	for res*res > n {
+		res = (res + n/res) / 2
+	}
+	divider := 2
+	for divider <= res {
+		if n%divider == 0 {
+			return false
+		}
+		divider++
+	}
+	return true
+}