有一系列產品，從某個開始其後都不合格，給定一個判斷是否合格的函數，找出N個產品中第一個不合格的產品。

正確答案：

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

class Solution {
public:
    int firstBadVersion(int n) {
        int low = 1;
        int high = n;
        int ver = 0;
        while (low < high) {
            ver 
 
= low + (high - low) / 2;   
            //這個題註意點！！如果直接high+low可能溢出，就會超時……
            
            if (isBadVersion(ver)) {
                high = ver;
            } else {
                low = ver + 1;
            }
        }
        return low;
    }
};

=============

有一道非常類似的題目

猜數遊戲：1-n之內有一個數是預選的數字，每次回告訴大還是小或者正確。返回猜中的數字。

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

class Solution {
public:
    int guessNumber(int n) {
        int start = 1;
        int end = n;
        int res = 1;
        while (start<end){
            
 
int mid = start + (end-start)/2;
            if (guess(mid)==0){
                return mid;
            }
            if (guess(mid)==1)
                start = mid+1;
            else
                end = mid-1;
        }
        if (guess(start))
            return start;
        return end;
    }
};

【easy】278. First Bad Version