[Swift]LeetCode479. 最大回文數乘積 | Largest Palindrome Product

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Find the largest palindrome made from the product of two n-digit numbers.

Since the result could be very large, you should return the largest palindrome mod 1337.

Example:

Input: 2

Output: 987

Explanation: 99 x 91 = 9009, 9009 % 1337 = 987

Note:

The range of n is [1,8].

---

 

你須要找到由兩個 n 位數的乘積組成的最大回文數。

因爲結果會很大，你只需返回最大回文數 mod 1337獲得的結果。

示例:

輸入: 2

輸出: 987

解釋: 99 x 91 = 9009, 9009 % 1337 = 987

說明:

n 的取值範圍爲 [1,8]。

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440ms

 1 class Solution {
 2     func largestPalindrome(_ n: Int) -> Int {
 3         var upper:Int = Int(pow(10.0,Double(n))) - 1
 4         var lower:Int = upper / 10
 5         let num:Int = lower + 1
 6         for i in (num...upper).reversed()
 7         {
 8             var str:String = String(i)
 9             var p:Int = Int(str + String(str.reversed()))!
10             var j:Int = upper
11             while(j * j > p)
12             {
13                 if p % j == 0
14                 {
15                     return p % 1337
16                 }
17                 j -= 1
18             }
19         }
20         return 9
21     }
22 }

---

8ms

1 class Solution {
2     func largestPalindrome(_ n: Int) -> Int {
3         return [9, 987, 123, 597, 677, 1218, 877, 475][n-1]
4     }
5 }

---

440ms

 1 class Solution {
 2     func largestPalindrome(_ n: Int) -> Int {
 3         if n==1  {return 9}
 4         var max = UInt64(pow(Double(10), Double(n))) - 1
 5         var min = max/10
 6         
 7         
 8         for v in stride(from: max, to: min, by: -1) {
 9             var ul = String(String(v).reversed())
10             var ur = String(v) + ul
11             var u = UInt64(ur)
12             var x = max
13             while x*x >= u! {
14                 if u! % x == 0 {return Int(u! % 1337)}
15                 x -= 1
16             }
17         }
18         
19         return 9
20     }
21 }