# 171. Excel Sheet Column Number

Given a 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: "A"
Output: 1
```

**Example 2:**

```
Input: "AB"
Output: 28
```

**Example 3:**

```
Input: "ZY"
Output: 701
```

```cpp
int titleToNumber(string s) { // time: O(n); space: O(1)
    int res = 0;
    for (char c : s) {
        res = res * 26 + (c - 'A' + 1);
    }
    return res;
}
```
