123 ---> 321
2147483647 --> 0; 翻转后溢出了
class Solution { public: int reverse(int x) { int ans = 0; while (x) { if (x > INT_MAX / 10 || x < INT_MIN / 10) return 0; ans = (ans * 10) + x % 10; x /= 10; } return ans; } };