Leetcode 7 Reverse Integer and 8 String to Integer (atoi)

Leetcode 7 Reverse Integer and 8 String to Integer (atoi)

This two problem share lots of similarities in data manipulating and thinking, and simplicity (:-D). Thus I decide to put them together. First let me get two Reverse Integer problem.

Edit Reverse Integer on GitHub

The description and example of problem is as follow:

```Example1: x = 123, return 321
Example2: x = -123, return -321```

Basically there are two tricks in this problem:

1. For a integer like 1000 that ends with one or more zeros, we should ignore certain zeros after reverse.

2. As we all know, integer has it’s limit, just as everything else (:-D). For a 32-bit signed binary integer, the maximum positive value is 2^31-1 = 2147483647. The minimum negative value is -2^32 = -2147483648. Thus, if the original integer is 1000000003, then the reverse will be 3000000001, which will cause a overflow issue.

After simple analysis, we could get to the simplest part:

```public class Solution {
public int reverse(int x) {
int isPositive = 1;
double sum = 0;
if (x<0){
isPositive=-1;
x=isPositive*x;
}
while(x>0){
int temp=(int)(x/10);
sum=sum*10+(x-10*temp);
x=temp;
if(sum>=2147483647) return 0;
if(sum<=-2147483648) return 0;
}
return isPositive*(int)sum;
}
}
```

The number manipulation is basic. First we use a flag  `isPositive` to determine if the number is positive or negative. For convenience, we convert negatives to positive, and simply convert it back after reverse.

Take the number 123 as example. We just have to take the number in least significant order, which is 3 in this case, and use it to construct the reverse number. This part is `int temp=(int)(x/10); sum=sum*10+(x-10*temp);` .

After first loop, we get `sum = 3; x = 12;`

After second loop, we get `sum = 32; x = 1;`

After final loop, we get `sum = 321; x = 0;` and exit the loop since `x = 0`

During the loop, we should also have overflow protection, which is `if(sum>=2147483647) return 0; if(sum<=-2147483648) return 0;`

Edit String to Integer (atoi) on GitHub

This problem is to implement a `atoi` function. Don’t tell you haven’t used a atoi function. There are several traps:

1. Since we are having `String` as input, we should assume that any characters could be input. We will need validation for those. The function `String.charAt()` is convenient to get each characters in string for validation. Thank got that integers from 0 to 9 is consecutive in ASCII, so that we can use `String.charAt(i)>'9'||String.charAt(i)<'0'`

For example, `^&*110` should return 0, `110&*^`  should return 110 (the custom is to ignore all the characters after the left most invalud charater).

2. There might be spaces in front of or at the end of the inputs, we should trim those. For example,  ` 1001 `  should return 1001. This can be done with `String.trim()`

3. Then we should consider the situation for signed integer. Thus we should have validation for string after trim to determine if the left most character is ‘+’ or ‘-‘ or valid number.

Now we get to simplest part again, the number manipulation is too straightforward to mention:

```public class Solution {
public int myAtoi(String str) {
String trim=str.trim();
if (trim==null||trim.length()==0){
return 0;
}
double sum=0;
int isPositive=1;

for(int i=0;i<trim.length();i++){
if(i==0&&(trim.charAt(i)=='+'||trim.charAt(i)=='-')){
if(trim.charAt(i)=='-'){
isPositive=-1;
}
continue;
}
if(trim.charAt(i)>'9'||trim.charAt(i)<'0') break;
sum=sum*10+trim.charAt(i)-'0';
}
sum=sum*isPositive;

return (int)sum;
}
}```