Excel table column number

1. Demand

• Given a column name in an Excel table, return its corresponding column number.

E.g,

    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

Two, base conversion

2.1 Thinking analysis

1. A~Z represents 1~26, traverse the string, get the current character must be one of A~Z, convert the current character into the corresponding integer, these integers are expressed in the base 26, which can be converted into base 10 ；

2.2 Code implementation

class Solution {
    public int titleToNumber(String s) {
        int sum = 0;
        for(int i = 0; i < s.length(); i++) {
            int tmp = s.charAt(i) - 64;
            sum = sum * 26 + tmp;
        }
        return sum;
    }
}

2.3 Complexity analysis

• The time complexity is O(N), and traversing the string consumes O(N) extra space;
• The space complexity is O(1);