feat(leetcode): add No.171

Author: mickey0524

171.Excel-Sheet-Column-Number.java

@@ -0,0 +1,24 @@
+// https://leetcode.com/problems/excel-sheet-column-number/
+//
+// algorithms
+// Easy (52.25%)
+// Total Accepted:    237,025
+// Total Submissions: 453,594
+// beats 100.0% of java submissions
+
+
+class Solution {
+    public int titleToNumber(String s) {
+        char[] chs = s.toCharArray();
+        int length = chs.length;
+
+        int base = 1, res = 0;
+        for (int i = length - 1; i >= 0; i--) {
+            int bit = (chs[i] - 'A') + 1;
+            res += bit * base;
+            base *= 26;
+        }
+
+        return res;
+    }
+}