45: triangular pentagonal

41_pandigital_prime/main.go

@@ -7,7 +7,7 @@ import (
 )
 
 func main() {
-	for i := 7654321; i > 1; i-- {
+	for i := 7654321; i >= 2143; i -= 2 {
 		if !isPandigital(strconv.Itoa(i)) {
 			continue
 		}
@@ -24,7 +24,6 @@ func isPrime(n int) bool {
 		return false
 	}
 	res := n
-	//牛顿法求平方根
 	for res*res > n {
 		res = (res + n/res) / 2
 	}

45_triangular_pentagonal_hexagonal/main.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+func main() {
+	i := 144
+	for {
+		m := 2*i*i - i
+		fmt.Println("i,m", i, m)
+		if isPentagonalNumber(m) {
+			fmt.Println("num:", m)
+			break
+		}
+		i++
+	}
+}
+
+func isPentagonalNumber(n int) bool {
+	r := math.Sqrt(float64(1 + 24*n))
+	if r-float64(int(r)) != 0 {
+		return false
+	}
+	return int(r)%6 == 5
+}
+
+func isTriangleNumber(n int) bool {
+	//tn = ½n(n+1)
+	x := 2 * n
+	res := x
+	//牛顿法求平方根
+	for res*res > x {
+		res = (res + x/res) / 2
+	}
+	for i := res; i > 0; i-- {
+		if i*(i+1) == x {
+			return true
+		}
+	}
+	return false
+}