[record of question brushing] 9. Number of palindromes

One 、 Title Description

source ：

Power button （LeetCode）

Give you an integer  x , If  x  Is a palindrome integer , return  true ; otherwise , return  false .

Palindrome number refers to positive order （ From left to right ） Reverse order （ From right to left ） Read all the same integers .

for example ,121  It's palindrome. , and  123  No .

Example  1：

Input ：x = 121
Output ：true


Example  2：

Input ：x = -121
Output ：false
explain ： Read left to right ,  by  -121 .  Read right to left ,  by  121- . So it's not a palindrome number .


Example  3：

Input ：x = 10
Output ：false
explain ： Read right to left ,  by  01 . So it's not a palindrome number .


Tips ：

-

Advanced ：

  Can you solve this problem without converting integers into strings ？

Two 、 Thought analysis

1.  Turn the string

If you don't consider advanced , The most direct way is to convert integers x Convert to string s1, Then directly use the function to reverse the string to s2, Same as the original string s1 Comparison is equal , You get the result .

2. Reverse the number

2.1  Reverse the whole number

Compared with the previous topic  

【 Record of writing questions 】7.  Integer inversion

  The idea in is similar to . Invert the integer and then compare it with the original number . Multiple judgments are   Negative numbers and  x%10 == 0  The array of cannot be palindromes

2.2  Invert half the array

The above method also needs to deal with the array overflow after inversion , We can optimize it a little bit .

Because if the number is palindrome , The reverse of the second half should be the same as the first half of the original number .

for example , Input  1221, We can put the numbers  “1221”  The second half of the  “21”  Reversal is  “12”,  And compare it with the first half  “12”  Compare , Because they are the same , We know the numbers  1221  It's palindrome. .

So we only need to get the second half of the palindrome , Compare with the previous one , The method of retrieving data is similar to that of inverting integers .

3、 ... and 、 Code implementation

class Solution {
 public boolean isPalindrome(int x) {
 //  Negative numbers and  10 Multiple of   Go straight back to
 if (x < 0 || (x % 10 == 0 && x != 0)) {
 return false;
 }

 int revertedNum = 0;
 while (x > revertedNum) {
 revertedNum = revertedNum * 10 + x % 10;
 x /= 10;
 }

 //  Palindrome number   There are two cases of length parity
 return x == revertedNum || x == revertedNum / 10;
 }
}


Complexity analysis

Time complexity ：

Spatial complexity ：

Running results

summary

This topic can actually be regarded as a kind of inversion , We only need to master one , Change again in form , In fact, they are not bad .