Create First Bad Version.md

Author: cheatsheet1999

JS-Algo/First Bad Version.md

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+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 returns whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.
+
+<img width="475" alt="Screen Shot 2021-11-13 at 22 06 30" src="https://user-images.githubusercontent.com/37787994/141668388-998ea209-0021-47e0-9803-27f878793d85.png">
+
+
+```JS
+/**
+ * Definition for isBadVersion()
+ * 
+ * @param {integer} version number
+ * @return {boolean} whether the version is bad
+ * isBadVersion = function(version) {
+ *     ...
+ * };
+ */
+
+/**
+ * @param {function} isBadVersion()
+ * @return {function}
+ */
+var solution = function(isBadVersion) {
+    /**
+     * @param {integer} n Total versions
+     * @return {integer} The first bad version
+     */
+    return function(n) {
+        let start = 1, end = n;
+        while(start < end) {
+            let mid = Math.floor((start + end) / 2);
+            if(isBadVersion(mid)) {
+                end = mid;
+            }
+            else {
+                start = mid + 1
+            }
+        }
+        return start
+    };
+};
+```