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;
}
}
```

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