chore(easy): add First Bad Version

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

Author: appleboy

easy/first-bad-version/README.md

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+# 278. First Bad Version
+
+See: [First Bad Version](https://leetcode.com/problems/first-bad-version/).
+
+You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the previous version, all the versions after a bad version are also bad.
+
+Suppose you have n versions `[1, 2, ..., n]` and you want to find out the first bad one, which causes all the following ones to be bad.
+
+You are given an API bool isBadVersion(version) which will return whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.
+
+Example:
+
+```sh
+Given n = 5, and version = 4 is the first bad version.
+
+call isBadVersion(3) -> false
+call isBadVersion(5) -> true
+call isBadVersion(4) -> true
+
+Then 4 is the first bad version.
+```

easy/first-bad-version/solution.go

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+/**
+ * Forward declaration of isBadVersion API.
+ * @param   version   your guess about first bad version
+ * @return 	 	      true if current version is bad
+ *			          false if current version is good
+ * func isBadVersion(version int) bool;
+ */
+
+func search(start, end int) int {
+		half := int((end - start)/2) + start
+		if !isBadVersion(half) {
+				return search(half+1, end)
+		}
+
+		if isBadVersion(half-1) {
+				return search(start, half)
+		}
+
+		return half
+
+}
+
+func firstBadVersion(n int) int {
+		return search(1, n+1)
+}