Reduce allocations and FfOrmatting

src/Julia/Problems001-050/Problem004.jl

@@ -10,9 +10,13 @@ https://projecteuler.net/problem=4 =#
 using BenchmarkTools
 
 
-function ispalindrome(n)
-    s = string(n)
-    s == reverse(s)
+ispalindrome(x::Integer, numdigits = ndigits(x)) = _ispalindrome(x, 10^(numdigits - 2))
+function _ispalindrome(x::Integer, divider)
+    x < 10 && return iszero(x) || iszero(divider)
+    (x, digit1) = divrem(x, 10)
+    (digit2, x) = divrem(x, divider)
+
+    return digit1 == digit2 && _ispalindrome(x, div(divider, 100))
 end
 
 
@@ -25,8 +29,9 @@ function Problem4()
 
     ans = 10_001
     for i = 100:999, j = i:999
-        if ((i * j > ans) && ispalindrome(i * j))
-            ans = i * j
+        num = i * j
+        if ((num > ans) && ispalindrome(num, ndigits(num)))
+            ans = num
         end
     end