class Solution {
public:
    int search(vector<int>& nums, int target) {
        int a = 0;
        int b = nums.size()-1,mid;
        while (b>a){
            mid = (a+b)/2;
            cout<<a<<" "<<b<<" "<<mid<<endl;
            if (nums[mid]==target) {
                cout<<"ret";
                return mid;
            }
            else if (nums[mid]>target){
                b = mid;
            }else{
                a = mid;
            }  
        }
        return -1;
    }
};

The output is :

Time Limit Exceeded
Stdout:
0 5 2
2 5 3
3 5 4
ret0 5 2
0 2 1
1 2 1
1 2 1
1 2 1

and 1 2 1 keeps repeating.
The test case is :
Input: nums = [-1,0,3,5,9,12], target = 9
Output: 4

The function has printed "ret", hence shouldn’t it return and stop? I tried this on my pc, works just fine. Is this a bug?
Leetcode didn’t give an error when it’s either b > a+1 or if instead of b = mid and a = mid, it’s b =mid-1 and a =mid+1.

>Solution :

Your code gets stuck in an infinite loop.

When a == 1 and b == 2 then mid = (1+2)/2 = 1 which makes your code take the a = mid = 1 branch and you never reach a == b.

The same happens whenever (a+b)/2 == a, ie when a + 1 == b.

The ret in the output is probably due to a second test case run by the online judge. For the first test your code returns a result but it then gets stuck on the second.