描述
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.
- The range of n is [1,8].
樣例
Input: 2
Output: 987
Explanation: 99 x 91 = 9009, 9009 % 1337 = 987
我認爲這個題有點蹊蹺,如果遍歷的話,會超時,然後翻看了大佬們的解法,最屌的解法叫做查表法,直接把答案列出來就好了。參考了大佬的解法:
http://www.cnblogs.com/grandyang/p/7644725.html
這種解法就是限制了一些條件範圍,製造了迴文數而不是判斷,第二步循環的時候,判斷的是平方數,因爲每次都是遞減,所以平方比製造的迴文數小的話,後面的數就不必驗證了。
class Solution {
public:
/**
* @param n: the number of digit
* @return: the largest palindrome mod 1337
*/
int largestPalindrome(int n) {
// write your code here
int upper = pow(10, n) - 1, lower = upper / 10;
for (int i = upper; i > lower; --i) {
string t = to_string(i);
long p = stol(t + string(t.rbegin(), t.rend()));
for (long j = upper; j * j > p; --j) {
if (p % j == 0) return p % 1337;
}
}
return 9;
}
};