KEY points:

  1. 7. Reverse Integer: overflow, Integer.MAX_VALUE
  2. 8. String to Integer (atoi): overflow, Integer.MAX_VALUE

better: 

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class Solution {
    public int reverse(int x) {
        int prev =0, res = 0;
        while (x != 0) {
            res = res * 10 + x % 10;
            if ((res - x % 10) / 10 != prev) return 0;
            prev = res;
            x /= 10;
        }
        return res;
    }
}

not so good:

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class Solution {
    public int reverse(int x) {
        long res = 0;
        while (x != 0) {
            res = res * 10 + x % 10;
            x /= 10;
            if (res > Integer.MAX_VALUE || res < Integer.MIN_VALUE) return 0;
        }
        return (int) res;
    }
}

The 2nd solution is not general because it’s using a long type. This is specifically for this one problem, i.e. reverse a 32-bit integer. What if it asks to reverse a 64-bit integer?: NOT GENERAL enough.

The better way is to check overflow instead of using larger data types.