278. 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.
Credits:Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

这个题目很容易就想到使用二分法来搜索：
当中点是Bad的时候，我们知道它后面就都是Bad，但是不知道这个是不是第一个bad，所以end=mid
当中点是Good的时候，我们知道这个元素和这个元素之前的所有元素都是Good的，所以head=mid+1
至于什么时候终止，你可以用只剩两个元素时的情况来测试一下。

/**
 * 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) {
        var head = 1;
        var end = n;
        while (head!==end) {
            var mid = parseInt(head + (end - head) / 2);
            if (isBadVersion(mid)) {
                end = mid;
            } else {
                head = mid+1;
            }
        }
        return head;
    };
};