[LeetCode August Challange]Day10-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

Constraints:

• 1 <= s.length <= 7
• s consists only of uppercase English letters.
• s is between “A” and “FXSHRXW”.

---

solution

Time complexity : O(n)
Space complexity : O(1)

class Solution {
public:
    int titleToNumber(string s) {
        unsigned int res = 0;
        for (int i=0; i<s.length(); ++i) {
            unsigned int base = pow(26, i);
            unsigned int num = calNum(*(rbegin(s)+i));
            res += base*num;
        }
        return res;
    }
private:
    unsigned int calNum(char c) {
        return c-'A'+1;
    }
};

從s的頭←尾處理，對於每一個字母：

1. 計算目前的位數次方，26^i。
2. 計算該字母對應的數值。
3. 以上兩者相乘後加入結果。