491. Palindrome Number【easy】

Check a positive number is a palindrome or not.

A palindrome number is that if you reverse the whole number you will get exactly the same number.

Notice

It's guaranteed the input number is a 32-bit integer, but after reversion, the number may exceed the 32-bit integer.

Example

11, 121, 1, 12321 are palindrome numbers.

23, 32, 1232 are not palindrome numbers.

解法一：

 1 class Solution {
 2 public:
 3     /*
 4      * @param num: a positive number
 5      * @return: true if it's a palindrome or false
 6      */
 7     bool isPalindrome(int num) {
 8         //negative number
 9         if(num < 0)
10             return false;
11             
12         int len = 1;
13         while(num / len >= 10)
14             len *= 10;
15             
16         while(num > 0)    {
17             
18             //get the head and tail number
19             int left = num / len;
20             int right = num % 10;
21             
22             if(left != right)
23                 return false;
24             else    {
25                 //remove the head and tail number
26                 num = (num % len) / 10;
27                 len /= 100;
28             }
29         }
30         
31         return true;
32     }
33 };

分别过滤出头尾2个数进行比较

解法二：

 1 class Solution {
 2 public:
 3     /*
 4      * @param num: a positive number
 5      * @return: true if it's a palindrome or false
 6      */
 7     bool isPalindrome(int num) {
 8         if (num < 0)  
 9             return false;  
10               
11         if (num < 10)  
12             return true;  
13               
14         int digits = 0;  
15         int t = num;  
16         int d = 0;  
17         while(t != 0) t /= 10, ++d;  
18           
19         int left = pow(10, d - 1);  
20         int right = 1;  
21         while( left >= right)  
22         {  
23             if (num / left % 10 != num / right % 10)  
24                 return false;  
25               
26             left /= 10;  
27             right *= 10;  
28         }  
29         return true;  
30     }
31 };

依旧是过滤出头尾2个数进行比较，处理的方式和解法一稍有不同，参考@MagiSu 的代码