triangle.go

package main

import (
  	"fmt"
	"math"
	"strconv"
)

func main() {
	smallest, _ := strconv.ParseInt("aaaaaaaaa", 16, 0)
	firstTriangle := nThTriangle(smallest)

	for i := firstTriangle; true ; i++ {
		x := triangle(i)
		str := fmt.Sprintf("%x", x)

		if len(str) >= 9 && isAllLetters(str) {
			fmt.Println(x)
			break
		}
	}
}

func isAllLetters(s string) bool {
	for i := range s {
		if '0' <= s[i] && s[i] <= '9' {
			return false
		}
	}

	return true
}

func triangle(n int64) int64 {
	return int64(n * (n + 1) / 2);
}

func nThTriangle(x int64) int64 {
	return int64((math.Sqrt(float64(8*x+1)) - 1 ) / 2)
}