二分查找法

 代码

public int mySqrt(int x){

     int l = 0, r = x,ans = -1;
        while(l <= r){
            int mid = l + (r - l) / 2;
            if((long) mid * mid <= x){
                ans = mid;
                l = mid + 1;
            }
            else{
                r = mid - 1;
            }
        }
        return ans;
}