add solution to excel sheet column number (#121)

Author: verma-anushka

C++/Excel-Sheet-Column-Number.cpp

@@ -0,0 +1,36 @@
+
+//Problem Statement: 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 
+//     ...
+
+//Solution:
+class Solution {
+public:
+    int titleToNumber(string s) {
+        
+        int power=0, colnum=0;
+
+        while(!s.empty()) {
+
+            colnum += (s[s.size()-1] - 'A' + 1)*pow(26, power);
+            power++;
+            s=s.substr(0, s.size()-1);
+
+        }
+
+        return colnum;
+    }
+};
+
+//Complexity: O(n)