first-bad-version.cpp

// 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:
//
//
// 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. 
//


// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        int start = 1;
        int end = n;
        
        while (end > start + 1) {
            int current = start + (end - start)/2;
            if (isBadVersion(current)) {
                end = current;
            }
            else {
                start = current;
            }
        }
        
        if (isBadVersion(start)) {
            return start;
        }
        else {
            return end;
        }
    }
};