The ticky part for this question is :

mid == 0     num[mid] < num[mid+1]   start = mid + 1;

0 < mid < len num[mid-1] < num[mid] < num[mid+1] start = mid + 1;

0 < mid < len num[mid-1] > num[mid] > num[mid+1] end = mid - 1;

mid = l num[mid] < num[mid-1] end = mid - 1;

 

But after we do the comparison, we can simplify the condition:

mid == 0 ||  mid < length-1 && num[mid] < num[mid + 1]   start = mid + 1

mid == length-1 || mid > 0 && num[mid] > num[mid - 1] end = mid - 1

 

1 class Solution { 2 public: 3     int findPeakElement(const vector
     
       &num) { 4         int start = 0, end = num.size()-1, mid = 0; 5         while (start <= end) { 6             mid = (start + end)/2; 7             if ((mid == 0 || num[mid] > num[mid-1]) && 8                 (mid == num.size()-1 || num[mid] > num[mid+1])) { 9                 return mid;10             } else if (mid == 0 || (mid < num.size()-1 && num[mid] < num[mid+1])){11                 start = mid + 1;12             } else {13                 end = mid - 1;14             }15         }16     }17 };