9. Palindrome Number

Question:

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

Example 1:

Input: 121
Output: true

Example 2:

Input: -121
Output: false
Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.

Example 3:

Input: 10
Output: false
Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Thinking:

1. 使用ArrayList装载了所有的数字。

class Solution {
    public boolean isPalindrome(int x) {
        if(x < 0) return false;
        List l = new ArrayList();
        while(x/10 != 0){
            l.add(x % 10);
            x /= 10;
        }
        if(x % 10 != 0){
            l.add(x%10);
        }
        int len = l.size();
        for(int i = 0; i < len/2; i++){
            if(l.get(i) != l.get(len-i-1)){
                return false;
            }
        }
        return true;
    }
}

二刷

1. 将int数字转换成了字符串。
2. 判断字符串是不是回文。
3. 将int转换成String的方法： new Integer(x).toString()

class Solution {
    public boolean isPalindrome(int x) {
        if(x < 0) return false;
        String s = new Integer(x).toString();
        return checkPalindrome(s);
    }
    private boolean checkPalindrome(String s){
        int low = 0, high = s.length() - 1;
        while(low < high){
            if(s.charAt(low++) != s.charAt(high--)) return false;
        }
        return true;
    }
}