approach 1

pop and push digits & check before overflow

we can pop the last digit by using modulus operation.
before we push the digit to the answer, we should check if it could cause overflow.

C++ code 1

class Solution {
public:
    int reverse(int x) {
        int sign = x < 0? -1: 1;
        if (sign < 0 && x == INT_MIN) return 0;
        int res = 0;
        x *= sign;
        while (x) {
            if (res > INT_MAX / 10 || res == INT_MAX / 10 && (x % 10) > (INT_MAX % 10))
                return 0;
            res = res * 10 + (x % 10);
            x /= 10;
        }
        return sign * res;
    }
};

C++ code 2

class Solution {
public:
    int reverse(int x) {
        int res = 0;
        while (x){
            int t = x % 10;
            x /= 10;
            if (res > 0 && res > (INT_MAX - t) / 10) return 0;
            if (res < 0 && res < (INT_MIN - t) / 10) return 0;
            res = res * 10 + t;
        }
        return res;
    }
};