public int reverse(int x) {
int rev = 0;
while (x != 0) {
int pop = x % 10;
x /= 10;
if (rev > Integer.MAX_VALUE / 10 || (rev == Integer.MAX_VALUE / 10 && pop > 7)) {
return 0;
}
if (rev < Integer.MIN_VALUE / 10 || (rev == Integer.MIN_VALUE / 10 && pop < -8)) {
return 0;
}
rev = rev * 10 + pop;
}
return rev;
}
Reversing an integer is similar to reversing String. We get the last digit x % 10
and add it to rev
. We repeat this process until x
is 0. The two if statements are used to prevent integer overflows.
Time Complexity: O(log(x))
Extra Space: O(1)
A note on integer overflows
A integer occurs when the sum of rev * 10 + pop
is greater/less than the maximum value (Integer.MAX_VALUE
)/the minimum value(Integer.MIN_VALUE
). So we need the check the values of rev
and pop
.
- If
rev > Integer.MAX_VALUE/10
orrev < Integer.MIN_VALUE/10
,rev * 10 + pop
will overflow. - If
rev == Integer.MAX_VALUE / 10
,pop
must be less than or equal to 7 because2^32 - 1 % 10 == 7
. - If
rev == Integer.MIN_VALUE / 10
,pop
must be greater than or equal to -8 because-2^32 % 10 == 8
.
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