二分查找

class BinarySearch {
public:
    int getPos(vector<int> A, int n, int val) {
        // write code here
        int start=0, end=n;        
        while(start < end){
            int mid = (start + end)/2;
            if(A[mid] < val) start = mid +1;
            else end = mid ;
        }
        if(end == n || A[end] != val) return -1;
        else return end;
        
    }
};