Update excel-sheet-column-number.cpp

Author: kamyu104

C++/excel-sheet-column-number.cpp

@@ -15,11 +15,10 @@ class Solution {
 
 // Time:  O(n)
 // Space: O(1)
-
 class Solution2 {
 public:
     int titleToNumber(string s) {
-        return accumulate(s.begin(), s.end(), 0,
+        return accumulate(s.cbegin(), s.cend(), 0,
                           [](int sum, char c) { 
                               return sum * 26 + c - 'A' + 1; 
                           });