Excel Sheet Column Number

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171.Excel Sheet Column Number

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

For example:

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

Example 1:

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Input: columnTitle = "A"
Output: 1

Example 2:

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Input: columnTitle = "AB"
Output: 28

Example 3:

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Input: columnTitle = "ZY"
Output: 701

时间复杂度：O(n)

空间复杂度：O(1)

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class Solution {
public:
	int titleToNumber(string columnTitle) {
		long n = pow(26, (columnTitle.size() - 1));
		int ans = 0;
		for (char c : columnTitle) {
			ans += (c - 'A' + 1) * n;
			n /= 26;
		}
		return ans;
	}
};