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.
Analysis:
注意有可能第一个就是坏版本。

Source Code（C++）:

#include <iostream>
#include <vector>
#include <cmath>
#include <string>
using namespace std;

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

class Solution {
public:
    int firstBadVersion(int n) {
        if (n<2){
            return 1;
        }
        if (isBadVersion(1)){
            return 1;
        }
        if (!isBadVersion(n)){  //全为好版本
            return 0;
        }
        int head=1, tail=n; //认为head是好，tail是坏版本
        while(tail-head>1) {
            int mid=(tail-head)/2+head;
            if (isBadVersion(mid)){
                tail=mid;
            }
            else {
                head=mid;
            }
        }
        return tail;
    }
};

int main() {
    Solution sol;
    cout << sol.firstBadVersion(1);
    cout << sol.firstBadVersion(2);
    cout << sol.firstBadVersion(10);
    return 0;
}

bool isBadVersion(int version) {
    if (version>1){
        return true;
    }
    else {
        return false;
    }
}