9. Palindrome Number

Description

YDetermine 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.

Follow up:

Coud you solve it without converting the integer to a string?

分析和方案

回文判断：字符串倒序排列之后是否和原字符串相同.

回文亦称回环，是正读反读都能读通的句子，亦有将文字排列成圆圈者，是一种修辞方式和文字游戏。回环运用得当，可以表现两种事物或现象相互依靠或排斥的关系。

例：

喜欢的少年是你，你是年少的欢喜

Madam, I'm Adam.

/**
 * @param {number} x
 * @return {boolean}
 */
var isPalindrome = function(x) {
    return String(x).split('').reverse().join('') === String(x);
};

对javascript来说，字符倒叙之后判断一下是否相同是很容易的。不过这个解法效率并非最好

最快

/**
 * @param {number} x
 * @return {boolean}
 */
var isPalindrome = function(x) {
    let num = x;
    let reversedNum = 0;
    while(num > 0) {
        let remainder = num % 10;
        num = (num - remainder) / 10;
        reversedNum = reversedNum * 10 + remainder;

        if (reversedNum === x) return true;
    }
    return x === 0;
};

从数字最低位按位取值，每次先*10再加取到的值，实际上做到了把数字倒序排列成新数值。最后判断是否和原值相同。

第二

/**
 * @param {number} x
 * @return {boolean}
 */
var isPalindrome = function(x) {
  if (x < 0) return false;
  if ((x / 10) >> 0 === 0) return true;
  if (x % 10 === 0) return false;

  let revertedNumber = 0;

  while (x > revertedNumber) {
    revertedNumber = revertedNumber * 10 + x % 10
    x = (x / 10) >> 0
  }

  return x === revertedNumber || x === (revertedNumber / 10) >> 0;
};

可能是入口if判断比较多，拖慢了速度，核心思路和最快类似

第三

var isPalindrome = function(x) {

    if(x<0){
        return false
    }else if(x>=0 && x<9){
        return true
    }

    var d = []

    num = x
    while(num>0){
        d.push(num%10)
        num = ~~(num/10)
    }

    var i = 0, j = d.length - 1
    while(i < j){
        if(d[i] == d[j]){
            j--
            i++
        } else {
            return false
        }
    }
    return true
}

对数据做了预判