finish 171

Author: BaffinLee

101-200/171. Excel Sheet Column Number.md

@@ -0,0 +1,76 @@
+# 171. Excel Sheet Column Number
+
+- Difficulty: Easy.
+- Related Topics: Math.
+- Similar Questions: Excel Sheet Column Title.
+
+## Problem
+
+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 
+    ...
+```
+
+**Example 1:**
+
+```
+Input: "A"
+Output: 1
+```
+
+**Example 2:**
+
+```
+Input: "AB"
+Output: 28
+```
+
+**Example 3:**
+
+```
+Input: "ZY"
+Output: 701
+```
+
+## Solution
+
+```javascript
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var titleToNumber = function(s) {
+  var res = 0;
+  var num = 0;
+  var len = s.length;
+  for (var i = 0; i < len; i++) {
+    num = getNum(s[len - 1 - i]);
+    res += Math.pow(26, i) * num;
+  }
+  return res;
+};
+
+var getNum = function (char) {
+  var start = 'A'.charCodeAt(0) - 1;
+  return char.charCodeAt(0) - start;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).