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. bool isBadVersion(int version);
bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        int i = 1;
        int j = n;
        int k = (i + (j - i) / 2);
        while (i < j) {
            if (isBadVersion(k)) {
                j = k - 1; // the answer is before k
            } else {
                i = k + 1; // the answer is after k
            }
            k = (i + (j - i) / 2);
        }
        return isBadVersion(i) ? i : i + 1;
    }
};