答案

public class Solution
{
    public int sqrt(int x)
    {
        if (x == 0)
        {
            return 0;
        }
        if (x < 4)
        {
            return 1;
        }
        int n=15;
        long result=0;
        long maxNum;
        for(;n>=0;n--){
            maxNum=1<<n;
            if((result+maxNum)*(result+maxNum)<=x){
                result+=maxNum;
            }
        }
        return (int)result;
    }
}