Given a string columnTitle that represents the column title as appear in an Excel sheet, return its corresponding column number.

For example:

```
A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28
...
```

Example 1:

```
Input: columnTitle = "A"
Output: 1
```

Example 2:

```
Input: columnTitle = "AB"
Output: 28
```

Example 3:

```
Input: columnTitle = "ZY"
Output: 701
```

Constraints:

1 <= columnTitle.length <= 7

columnTitle consists only of uppercase English letters.

columnTitle is in the range ["A", "FXSHRXW"].

Solution:

```
class Solution {
public int titleToNumber(String columnTitle) {
if(columnTitle==null) return -1;
int sum=0;
for(char c: columnTitle.toCharArray())
{
sum*=26;
sum+=c-'A'+1;
}
return sum;
}
}
```