9. 回文数 – Blog of Aspirer

题目：https://leetcode.cn/problems/palindrome-number/

这里是先把每个数字取出来存到int[]，然后while循环两头比对数字是否相同。也可以先用itoa转换成char[]类型，再strlen看长度，最后while循环两头比对字符是否相同，也是类似的逻辑。

#include <iostream>

bool isPalindrome(int x) {
    if (x < 0) {
        std::cout << x << ": false" << std::endl;
        return false;
    }
    if (x < 10) {
        std::cout << x << ": true" << std::endl;
        return true;
    }

    int in = x;
    int num[10] = {0};
    int index = 0;
    int i = 0;

    // store every number of x to an int array in the reverse order
    while (x >= 10) {
        num[index++] = x % 10;
        x = x / 10;
    }
    num[index] = x;

    // print the array
    std::cout << in << ": ";
    while (i <= index) {
        std::cout << num[i++] << " ";
    }
    std::cout << std::endl;

    // check the array is palindrome or not
    i = 0;
    while (i < index) {
        if (num[i++] != num[index--]) {
            std::cout << in << ": false" << std::endl;
            return false;
        }
    }

    std::cout << in << ": true" << std::endl;
    return true;
}

int main() {
    std::cout << "-------- false --------" << std::endl;
    isPalindrome(-12);
    isPalindrome(10);
    isPalindrome(120);
    isPalindrome(1112);
    isPalindrome(11212);
    isPalindrome(1021);
    isPalindrome(10021);
    isPalindrome(100021);
    isPalindrome(1000021);
    isPalindrome(10000201);
    isPalindrome(100002001);
    isPalindrome(1000020001);

    std::cout << "-------- true --------" << std::endl;
    isPalindrome(0);
    isPalindrome(1);
    isPalindrome(11);
    isPalindrome(101);
    isPalindrome(111);
    isPalindrome(12121);
    isPalindrome(21112);
    isPalindrome(21212);
    isPalindrome(100020001);
    return 0;
}

#include <iostream>

bool isPalindrome(int x) {

if (x < 0) {

std::cout << x << ": false" << std::endl;

return false;

}

if (x < 10) {

std::cout << x << ": true" << std::endl;

return true;

}

int in = x;

int num[10] = {0};

int index = 0;

int i = 0;

// store every number of x to an int array in the reverse order

while (x >= 10) {

num[index++] = x % 10;

x = x / 10;

}

num[index] = x;

// print the array

std::cout << in << ": ";

while (i <= index) {

std::cout << num[i++] << " ";

}

std::cout << std::endl;

// check the array is palindrome or not

i = 0;

while (i < index) {

if (num[i++] != num[index--]) {

std::cout << in << ": false" << std::endl;

return false;

}

std::cout << in << ": true" << std::endl;

return true;

}

int main() {

std::cout << "-------- false --------" << std::endl;

isPalindrome(-12);

isPalindrome(10);

isPalindrome(120);

isPalindrome(1112);

isPalindrome(11212);

isPalindrome(1021);

isPalindrome(10021);

isPalindrome(100021);

isPalindrome(1000021);

isPalindrome(10000201);

isPalindrome(100002001);

isPalindrome(1000020001);

std::cout << "-------- true --------" << std::endl;

isPalindrome(0);

isPalindrome(1);

isPalindrome(11);

isPalindrome(101);

isPalindrome(111);

isPalindrome(12121);

isPalindrome(21112);

isPalindrome(21212);

isPalindrome(100020001);

return 0;

}