bool binarySearch(int n,int x){
    //查到返回true
    int left = 0;
    int right= n-1;

    while(left <= right){

        int mid = (left + right) / 2;

        if(a[mid] == x) return true;

        //从小到大排列，中间值大于查找值
        else if(a[mid] > x) right = mid -1;

        else left = mid + 1;

    }
    return false; //查不到返回false
}