Reverse Integer: pop and push

• Pop: take the last digit out by using x % 10
• Push:push this digit back by rev * 10 + digit

Overflow Handling

• Make sure the result is not larger than INT_MAX or smaller than INT_MIN
• Compare them BEFORE push back digits

if (rev > INT_MAX / 10 || (rev == INT_MAX / 10 && digit > 7 ))

or

if (rev < INT_MIN / 10 || (rev == INT_MIN / 10 && digit < -8))