LintCode刷题---x的平方根

    public int sqrt(int x) {
        // write your code here
        if(x<0) throw new IllegalArgumentException();
        else if(x<=1){
            return x;
        }
        int start = 1;
        int end = x;
        while(start+1<end){
            int mid = start + (end - start)/2;
            if(mid == x/mid){
                return mid;
            }
            else if(mid > x/mid){
                end = mid;
            }
            else{
                start = mid;
            }
        }
        if(end>x/end){
            return start;
        }
        return end;
    }