278. First Bad Version

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problem

solution

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// The API isBadVersion is defined for you.
// bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        int l =1, r = n;
        while(l<=r){
            int mid = l + (r-l)/2;
            if(isBadVersion(mid))  r= mid-1;
            else if(!isBadVersion(mid)) l = mid+1;
        }
        return l;
        
    }
};
  • lower bound
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    // The API isBadVersion is defined for you.
    // bool isBadVersion(int version);

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

    analysis

  • time complexity O(logn)
  • space complexity O(1)