Leetcode 278 First Bad Version 二分查找（二分下标）

题意：找到第一个出问题的版本

二分查找，注意 mid = l + (r - l + 1) / 2;因为整数会溢出

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

class Solution {
public:
    int firstBadVersion(int n) {
        int l = 1, r = n , ans ;
        while(l <= r){
            int mid = l +  (r - l + 1) / 2;
            if(!isBadVersion(mid)){
                l = mid + 1;
            }
            else {
                r = mid - 1;
                ans = mid;
            }
        }
        return ans;
    }
};